Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/21/17
Area Forecast Discussion
National Weather Service Columbia SC
1012 PM EST Wed Dec 20 2017
.SYNOPSIS...
A cold front will sink south of the forecast area tonight. It
will remain cloudy and cool just behind the front Thursday.
Expect mild temperatures through this weekend ahead of the next
frontal system. Dry and cooler conditions will return early next
week behind the front.
&&
.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
The cold front will sink south of the forecast area tonight. The
RAP and satellite trends indicate a mid-level shortwave trough
will shift east of the forecast area around 100 AM. The HRRR
displays areas of rain shifting east of the area about this
time. Low cloudiness and possibly fog will occur overnight.
However, fog should be limited by wind. There may be areas of
drizzle in the moist low-level flow. Cloudiness and mixing
favor the higher low temperature guidance.
&&
.SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY NIGHT/...
The upper-level trough and bulk of the moisture will be east of
the area Thursday. Northeast winds will bring cool, dry air
beneath a shallow layer of moisture and mid-level subsidence.
The low-level inversion will remain in place through much of
the day keeping daytime temperatures cool. Synoptic-scale
subsidence and a thin cloud layer suggests rain or even drizzle
is unlikely. Daytime temperatures will likely remain in the
lower 50s to mid 50s. The cloud cover may break up a few hours
before sunset which would allow for some additional warming.
Mostly cloudy skies should keep overnight lows in the low to mid
40s.
Models continue to develop an upper-level trough in the central
CONUS Friday, with an upper ridge building over the SE US.
Expect the low-level inversion to gradually erode Friday,
allowing temperatures to warm up into the 60s with weak
southerly flow developing. A cold front associated with the
upper trough will approach the Southeast Friday night, however
rainfall will likely remain west of the forecast area.
&&
.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
The upper-level trough will shift into the E CONUS by Sun/Mon
allowing the cold front to slowly move through the forecast
area. The greatest chance for rain will be Saturday night and
Sunday. Cool, dry high pressure expected to build in for early
next week. Temperatures will likely be below normal for
Christmas day through the remainder of the long term.
&&
.AVIATION /03Z THURSDAY THROUGH MONDAY/...
High confidence in widespread restrictions through the forecast
period.
Latest SPC HRRR indicting that the precipitation will move east
of the terminals around 06Z-07Z. IFR cigs already developing
across the area with little improvement overnight. Expect
northeast winds and wedge like conditions to take hold with
more widespread IFR/possible brief LIFR restrictions in
vsby/cigs through the end of the period for the rest of the
overnight hours, with IFR conditions continuing after sunrise
Thursday towards the end of the TAF period.
EXTENDED AVIATION OUTLOOK...Improving conditions by early
Thursday evening. The next low pressure system will move
through the region Friday afternoon through Sunday with chances
of rain and associated restrictions.
&&
.CAE WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
&&
$$
Area Forecast Discussion
National Weather Service Austin/San Antonio TX
816 PM CST Wed Dec 20 2017
.UPDATE... /ADDED FOG THU NITE PLUS MINOR PRECIP TREND DETAILS/
Prefrontal conditions for Thursday night should be mainly some light
showers, but the approach of the front toward daybreak Friday could
lead to a few lightning strikes over the northern Hill Country. Also
updated to mention some prefrontal patchy fog early Friday considering
that the moist low levels and light precipitation will be accompanied
by a weak pressure gradient.
&&
.PREV DISCUSSION... /ISSUED 558 PM CST Wed Dec 20 2017/
UPDATE...
00Z Aviation update below.
AVIATION...
VFR conditions at TAF sites are expected tonight with SW to SE winds
less than 5 kts. Patchy ground fog is possible east of I-35
overnight. HRRR is suggestion more widespread fog developing across
the coastal plains 09Z-12Z and advecting north into inland areas of
South Texas 12Z-15Z. This could potentially reach some inland non-TAF
sites such as PEZ and T20 during this time with LIFR to IFR ceilings
and visibility. At this time am forecasting low MVFR deck to develop
15Z-18Z at SAT/SSF closer in line with BUFR soundings, and spreading
north toward AUS by 21Z. A S to SE winds 5-10 kts is expected to
develop Thu afternoon. Conditions will deteriorate Thu evening as
widespread IFR to potentially LIFR ceilings develop.
PREV DISCUSSION... /ISSUED 244 PM CST Wed Dec 20 2017/
SHORT TERM (Tonight through Thursday Night)...
An upper level trough currently over the northwestern part of the
country will drop down the west coast and begin to move east through
the period and turn the flow over Texas to the southwest by Thursday
evening. High pressure at the surface will move to the east bringing
moist low level air back to region. Patchy fog will form again
tonight across our southern CWA. An upper level short wave trough
will move through the pattern and bring a chance for rain Thursday
night.
LONG TERM (Friday through Wednesday)...
Shower activity will increase Friday morning and afternoon as a cold
front moves through Central Texas. Isolated thunderstorms will be
possible as the front moves into our CWA during the afternoon hours
with some weak surface based and elevated instability in place.
Showers should continue into the overnight hours as the upper level
trough passes through to help maintain isentropic ascent at mid-levels
behind the front. Cooler air will move in behind the front with
cooler temperatures for the weekend. The coldest morning will be
Monday when most of the area will have freezing temperatures. Tuesday
the low level flow will turn back around to the south and southeast
bringing moister, warmer air back to the region. This return of
moisture will bring a slight chance for rain again Wednesday.
&&
.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry 53 77 63 67 41 / 0 10 40 80 50
Austin Bergstrom Intl Airport 49 76 63 68 41 / 0 10 40 70 50
New Braunfels Muni Airport 51 75 63 69 43 / 0 10 40 60 50
Burnet Muni Airport 49 77 56 58 37 / 0 - 50 80 50
Del Rio Intl Airport 48 75 59 66 42 / 0 - 20 30 20
Georgetown Muni Airport 49 77 59 61 40 / 0 - 50 80 50
Hondo Muni Airport 47 75 62 71 43 / 0 - 30 50 50
San Marcos Muni Airport 50 75 63 70 42 / 0 10 40 70 50
La Grange - Fayette Regional 51 75 65 73 45 / 0 20 40 60 60
San Antonio Intl Airport 52 74 63 70 44 / 0 10 40 60 50
Stinson Muni Airport 51 74 65 72 46 / 0 10 30 50 50
&&
.EWX WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
Mesoscale/Aviation...Runyen
Synoptic/Grids...Oaks
Public Service/Data Collection...33
Area Forecast Discussion
National Weather Service Grand Forks ND
737 PM CST Wed Dec 20 2017
.UPDATE...
Issued at 731 PM CST Wed Dec 20 2017
A heavier snow band laying across far southern Grand Forks and
northern Steele and Traill County line extends east southeast into
Minnesota with heavy snow likely from Buxton-Climax areas across
Mahnomen-Naytahwaush into Two-Inlets and Park Rapids areas.
Lighter snow accumulations are still expected outside that area...
with snow diminishing from the west after midnight.
UPDATE Issued at 645 PM CST Wed Dec 20 2017
Light snow over much of the area ATTM with a narrow band of
heavier snow across the central part of the forecast area. This
heavier band should steadily shift eastward through the evening
with snowfall tapering off in eastern ND by midnight but
persisting in northwest MN through early morning. An update
Aviation Discussion is attached below.
&&
.SHORT TERM...(This evening through Thursday)
Issued at 312 PM CST Wed Dec 20 2017
Snow amounts for tonight are the main issue for the period.
Snow has intensified across the central CWA during the past few
hours, which matches well with the strong 700mb frontogenesis
band as seen on SPC meso page. There is also good isentropic lift
across much of the CWA with broad lift across the region bringing
light snow beyond the strongest snow band. The short range models
have this heavier band weakening later this afternoon and into the
evening, which fits the brief decrease in 700mb frontogenesis
that the larger scale models show. The NAM tries to develop more
frontogenesis later on tonight, but there is less larger scale
lift than would be ideal for banding, and the 12 and 18Z NAM runs
have been all over the place with the second round of moderate
snow. The RAP and HRRR have been pretty consistent with 4 to 6
inches of accumulation pretty close to where we have been calling
for it, lower Devils Lake Basin into the central Red River Valley
and off into west central MN. With a slight trend further south,
included Wadena county in the advisory. As the high resolution
models are fairly consistent, will keep the rest of the advisory
as it is. Although there could be less snow along the northern and
southern edges of where we have the heaviest band, there is
enough uncertainty as to where the second snow band will set up to
start cutting counties out.
The snow will start to taper off across the northern counties by
midnight, and across the south by early morning. Some clearing
will be possible across the far northwestern CWA by tomorrow
morning, and with fresh snow temps will plummet into the 5 to 10
below zero range. Dry and quiet for tomorrow between systems, with
highs in the single digits and teens.
.LONG TERM...(Thursday night through Wednesday)
Issued at 312 PM CST Wed Dec 20 2017
Friday and Saturday...Generally quiet weather will prevail with
the main if not only feature swinging through Fri in the form of
a cold front with minimal QPF. Temps in advance of this front will
be the warmest they will be for a quite a length of time with
highs between 15 and 25. Arctic air is slated to plunge southward
later Sat into Sun with 850 mb temps dropping to minus 25 C.
Sunday through Wednesday...By Sun night those 850 temps will drop
toward -30 C. Absolute minima will always hinge upon freshness of
snowpack, cloud cover and wind speeds but it will not be
difficult to reach double digits below zero region wide Sun and
Mon nights with a shot at minus 25 over parts of NW MN. This will
easily create wind chill impacts well into advisory criteria for
Christmas eve and day. The potential for a 48 hour period of sub
zero temps exists for the area along and north of Highway 2.
Little if any pcpn is expected during the early to middle part of
next week.
&&
.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 645 PM CST Wed Dec 20 2017
Expect widespread MVFR CIGS and VSBYS in light snow across the FA
throughout the overnight period...with areas of IFR CIGS and VSBYS
across central portions of the FA. From 08z through 12z expect
conditions to slowly improve to VFR across eastern ND and the Red
River Valley. From 12z through 15z expect conditions to slowly
improve to VFR across northwest MN.
&&
.FGF WATCHES/WARNINGS/ADVISORIES...
ND...Winter Storm Warning until 2 AM CST Thursday for NDZ027-029-030.
Winter Weather Advisory until 6 AM CST Thursday for NDZ006-
014>016-024-026-028-038-039-054.
MN...Winter Storm Warning until 6 AM CST Thursday for MNZ022>024-027-
028.
Winter Storm Warning until 2 AM CST Thursday for MNZ001-002.
Winter Weather Advisory until 6 AM CST Thursday for MNZ003-
007>009-013>017-032.
&&
$$
UPDATE...Gust
SHORT TERM...JR
LONG TERM...WJB
AVIATION...Gust
Area Forecast Discussion
National Weather Service Flagstaff AZ
859 PM MST Wed Dec 20 2017
.SYNOPSIS...A cold front will move into northern Arizona this
evening. Expect strong southwest winds tonight into Thursday
morning, and light rain and snow showers from late tonight through
Thursday afternoon. Chances for showers are forecast for
locations along and north of the Mogollon Rim, with the best
chances north of Interstate 40. High temperatures Thursday look to
be in the low 30s in the mountains to around 50 in the Verde
Valley. Warmer temperatures and dry weather are forecast for the
weekend.
&&
.UPDATE...
9 pm surface obs show the cold front along a line from Hanksville
UT to Kanab to near Daggett CA. Area radars show a band of rain
and snow showers now entering Coconino county. Areas where the
band is are showing hourly precip in the .01-.05" range. Snow
levels to fall to around 5000 feet on the Kaibab Plateau by
midnight tonight.
Strong S-SW winds are still expected overnight across
areas along and north of the Mogollon rim from Flagstaff through
Springerville, and then northward towards the Little Colorado
River. This due to a strengthening of the low level winds ahead
of the cold front. I-40 at Two Guns has been gusting to around 50
mph since 730 pm, and we should see more of this across southern
Navajo and Apache counties early Friday morning.
&&
.PREV DISCUSSION /535 PM MST/...
The cold front at 5 pm along a line from Milford Utah-Las Vegas
-Santa Barbara. The wind advisories out for tonight through early
Thursday morning look good. We have had several gusts in the 35
to 50 mph range this afternoon. We expect this plus another 5 mph
tonight for areas along and just north of the Mogollon rim as a
southwesterly low level jet blows across the rim. We have
increased and in some cases nearly doubled tonights wind gusts in
the wind advisory areas over those sent with the 330 pm Thursday
forecast package. The fastest winds for this event are forecast
to be from Show low through Springerville, with gusts to 60
possible. Doney Park not to far behind this.
Forecast updates have been sent. DL
From the 452 am Thursday Discussion...
By RR
Similarly, the surface cold front will cross the area from the
northwest to southeast, first impacting the Kaibab Plateau shortly
before midnight tonight and exiting the area by way of the White
Mountains Thursday afternoon. Chances for rain and snow showers
are forecast along and north of the Mogollon Rim, with the best
chances north of Interstate 40. In general, 1 to 2 inches of snow
are forecast along the Kaibab Plateau and mountains of far
northeast Arizona. Limited QPF and a saturated layer only up to
about -12 C should limit snow along the western Mogollon Rim.
Around 0.5 inches of snow is forecast in the Flagstaff area. In
addition, very cold air behind the front will keep daytime high
temperatures on Thursday between 10 to 15 degrees below normal
across much of the area.
Dry weather and warming conditions are forecast for Friday, with
near normal temperatures in place for this Holiday weekend. Models
generally agree that a ridge of high pressure will lead to above
normal temperatures by Tuesday of next week.
&&
.AVIATION...For the 06Z package...An approaching cold front will
bring a chance for rain and snow showers with localized MVFR
conditions to areas along and north of I-40 beginning 06Z-09Z, with
southwest wind gusts to around 35 to 45 kts continuing in the
meantime. Shower activity diminishes from northwest to southeast
during the daytime Thursday. Winds will shift to the north-northwest
behind the front. Aviation discussion not updated for TAF amendments.
&&
.FIRE WEATHER...Strong and gusty southwest winds will persist into
the overnight hours as a cold front approaches the region. This
front will bring a chance of rain and snow showers later this
evening and into Thursday mainly north of I-40. Behind the front,
winds will shift northerly and usher in the coldest airmass of the
season Thursday. Northeasterly winds will persist Friday with
daytime temperatures near normal.
Saturday through Monday...Above normal daytime temperatures return
for the weekend and early next week under dry west-northwest flow.
&&
.FGZ WATCHES/WARNINGS/ADVISORIES...Wind Advisory until 2 AM MST Thursday FOR AZZ012-013-015-016.
Wind Advisory until 11 AM MST Thursday FOR AZZ014-017.
&&
$$
PUBLIC...DL/RR
AVIATION...KD
FIRE WEATHER...TPS
For Northern Arizona weather information visit
weather.gov/flagstaff
Area Forecast Discussion
National Weather Service Houston/Galveston TX
531 PM CST Wed Dec 20 2017
.AVIATION...
Two main issues in this evening`s forecast are trends in winds,
and visibility. Pretty high confidence that southwesterly/westerly
winds from this afternoon will become light and variable
overnight. When they reestablish in the morning, look for
southerly winds.
Visibility and ceilings are a trickier bit. There had been strong
HRRR consensus for dense fog for the entire area from IAH
coastward towards dawn. Given that dewpoints in the area are still
only in the mid to upper 40s, without a real strong onshore
component to boost them quickly, was unsure of how that would play
out. Fortunately, the latest HRRR seems more in line with
observations. In short, towards dawn, have MVFR visibility from
CXO to SGR/HOU. Do go to high IFR at LBX and GLS, nearer the
coast. Finally, bring in some low MVFR ceilings all the way to UTS
(but leave out CLL) tomorrow afternoon as onshore flow and
moisture return kick in more strongly.
Luchs
&&
.PREV DISCUSSION... /ISSUED 319 PM CST Wed Dec 20 2017/
DISCUSSION...
Clear skies overnight with light and variable winds, should help
prime conditions for the potential development of radiational fog.
Patchy fog should develop where the best moisture will be
available, essentially from I-10 southward, as well as in our
western counties. The onset of development would begin around 09Z
Thursday and could carry through late morning. Short term guidance
such as the SREF and HRRR also indicates the potential for
lowered visibilities starting along the coast and slowly spreading
inland and westward.
Surface winds though light overnight, will begin to turn more
southerly early Thursday allowing a warm air advection pattern to
set up. This will help to push warm moist air back over the
region. As this airmass advects over the cooler shelf waters, the
chance for sea fog will return Thursday morning along the coastal
counties. Patchy to thicker areas of fog could linger through
much of Thursday afternoon. Cloud cover will also thicken as a
stratus deck develops early Thursday, and should keep temperatures
a touch cooler. High temperatures for Thursday are still above
normal for this time of year, in the mid to upper 70s. The chance
for sea fog will again be a factor early Friday morning out ahead
of our next cold front.
The global models have come into better agreement with the timing of
the next cold front, which should reach our coastal waters
between 06Z-12Z Saturday. Showers and isolated thunderstorms will
be possible with this frontal passage, with precip starting as
early as late Thursday and carrying through the early afternoon
hours Saturday. Showers can be expected out ahead of the front
late Thursday, with the bulk of the precip associated with the
frontal boundary occuring late Friday into early Saturday. Behind
the front, showers again will fill in, lingering into the early
afternoon hours on Saturday. Temperatures will not be as cool
behind this first front, as previous runs of the GFS and ECMWF
anticipated. As a result, high temperatures Friday compared to
Saturday will drop anywhere from 10 to 15 degrees, leaving high
temperatures Saturday in the upper 50s to lower 60s. A cooler
drier airmass will arrive late Sunday into Monday, which will
bring the secondary push of cooler air. This airmass will lower
temperatures a little further bringing Monday`s high temperatures
into the 50s. Therefore, slightly below normal temperatures are
in store for your holiday weekend.
The warming trend should continue through mid week, bringing
temperatures back into the upper 50s to mid 60s. Onshore flow
should return by Monday evening. Lastly, another cold front, is
expected to arrive towards the end of next week. This will result
in another round of precip beginning early Wednesday, with a front
potential sliding off the coast next Thursday.
Hathaway
MARINE...
Light onshore winds will develop tonight as surface high pressure moves
east of the region. A warm front and associated return flow will bring
deeper moisture back into the area and surface dew points will increase
to near 70 by Thursday afternoon. Water temperatures are in the lower
60s at the current time so would expect sea fog to begin to develop
perhaps as early as Thursday morning. Visibility will probably fall
to below a mile at times and a Marine Dense Fog Advisory may be needed at
times. One small concern regarding the potential for sea fog is that
surface winds will be S-SE and this will bring slightly warmer waters
toward the coast so the difference between the water temp and dew pt
will lessen and this could reduce the potential for sea fog. SREF ensembles
keep the best potential for sea fog from about Galveston Bay, eastward
along the Louisiana coast. The onshore flow will also strengthen Thursday
into Friday and a SCEC may be required for the 20-60 NM waters.
A cold front will cross the coast between 09-12z Saturday and usher in
an offshore flow and end the fog threat. Borderline SCA conditions expected
in the wake of the front. A secondary surge of colder air will enter the
area on Sunday and the gradient over the water will tighten with SCA conditions
expected over the Gulf waters on Sunday night into Monday. Onshore winds will
return by Monday evening and persist through next Wednesday. 43
&&
.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL) 51 77 64 70 44 / 0 10 40 60 70
Houston (IAH) 53 77 67 76 52 / 0 20 30 40 70
Galveston (GLS) 60 74 65 74 56 / 10 20 20 30 70
&&
.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...NONE.
&&
$$
Discussion...39
Aviation/Marine...25
Area Forecast Discussion
National Weather Service Wilmington NC
953 PM EST Wed Dec 20 2017
.SYNOPSIS...
Low pressure and its associated cold front will shift offshore
of the area tonight. A brief cool down will occur Thursday.
More showers and a return to unseasonably warm temperatures will
return this weekend ahead of another cold front. We should dry
out Christmas day with temperatures turning sharply colder as
Arctic air seeps into the eastern Carolinas, bringing the risk
for freezing temperatures early next week.
&&
.NEAR TERM /THROUGH THURSDAY/...
As of 1000 PM Wednesday...The second in a pair of shortwaves is
now plowing through the western Carolinas, lighting up another
batch of light to moderate rainfall along the Savannah River
through the Upstate of South Carolina and into the NC mountains
and foothills. This is a little more widespread than I`d earlier
anticipated, and I`ve increased PoPs through midnight into the
80-100% range for most areas. Among the hourly models, the HRRR
is doing a better job than the RAP, and is used as the basis for
this forecast update.
Mesonet precipitation totals through 930 PM are as high as 1.49
inches in the West Lakes neighborhood in Florence, 1.40 inches
near Pawley`s Island, and 1.28 inches 7 miles southeast of
Hemingway, SC.
Some minor upward tweaks to temperatures and dewpoints have
also been made through the night, but the biggest change was to
increase PoPs through midnight. Discussion from 300 PM
follows...
Cold front was basically stretched from west to east bisecting
our forecast area early this afternoon. Shallow cool air was
advecting in with northerly flow behind it. The temps to the
north of the boundary were in the mid 50s while temps were up
near 70 degrees about 70 miles to the south. At the same time a
low pressure system will track in from the west along this
boundary. Decent lift was supported by the warm and moist sw
flow over this shallow cool air along with increasing upper
level support as shortwave and strong jet traverse the area
through this afternoon into this evening. Radar was showing
fairly widespread moderate rain spreading east and south into
the I-95 corridor and expect this trend to continue. The rain
should cover most or all of the forecast area late this
afternoon into this evening. Expect upwards of an inch of rain
in places with periods of steady moderate rain through early
this evening. Pcp water values will reach up near 1.75 inches
across northeast SC early this eve. Overall expect a wet evening
across the area. More convective type pcp with embedded heavier
rainfall will move across SC through this afternoon. SPC
continues to include coastal Carolinas in general thunder in a
high shear/low CAPE environment. Looking at the sounding for
FLO, there is limited elevated CAPE into early this evening but
have not seen any lightning as of yet. Most of the lift will be
related to stronger isentropic lift along and north of the
front.
Winds will pick up out of the northeast as area of low pressure
shifts off the coast after midnight. Expect a shot of cool air
behind this front with temps dropping into the low to mid 40s by
daybreak in CAA. The 850 temps will drop about 5c within the
next 24 hours. The shortwave will clear the coast just before
daybreak with deeper drier air behind it. NAM moisture profiles
show a shallow dry and cool layer, but continue to show a layer
of moisture between 3k and 8k ft and this could lead to a cloudy
day for Thursday. The mid to upper levels above that will be
very dry as well. The GFS shows a greater amount of drying
through early aftn on Thurs which should produce a greater
amount of sunshine. For now will forecast mixed sun and cloud
for much of NC Thurs aftn but kept mostly cloudy for much of SC as
high pressure builds in from the north in a wedge type pattern. The
cloudier NAM forecast will keep temps closer to 50 while the more
optimistic GFS guidance will produce temps closer to 55. For now
will go with lower 50s for highs across much of the area Thurs
aftn.
&&
.SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
As of 300 PM Wednesday...Main weather caption this time frame,
"Patchy Light Coastal Rain Friday, then Warming Area-Wide". A
coastal warm front will lift northward Thursday night and
Friday, to the Outer Banks by Friday evening. This heralds the
onset of a notable warming trend progressing into the weekend.
Model, column-moisture profiles, shows 975-775 mb saturation
Thursday night and Friday, as the coastal warm front transits
northward, suggesting patchy light rain and drizzle along the
coast, and coastal interior of SE NC, but over-all limited QPF
numbers for this feature.
The coolest period, early Friday prior to warm air advection,
40-43 inland, 44-47 by the sea and ICW. Dewpoints will hold
mainly in the 40s, but look for them to rise into early Sat.
&&
.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
As of 300 PM Wednesday...Ridge of high pressure will shift
S and E of the area Sat. A series of shortwave troughs will
help to deepen an upper trough across the middle of the nation.
The last of these significant shortwaves will lift out across
the Northeast states Sun night and Mon. This will bring a slow
moving cold front across the eastern Carolinas late Sun night
and Mon morning. In the wake of this front, Arctic air will
build in from the W Mon with the center of the strong high
overhead on Tue. An expansive area of very strong high pressure
across eastern Canada will exert a southward push across the
Carolinas on Wed with a coastal trough possibly developing
offshore late.
We will be on a temperature roller coaster with weekend temps
well above normal and likely in the 70s away from the cooling
effects of the ocean on Sat. Mon will be transitioning to temps
well below normal Tue and Wed when highs may only be in the 40s.
Just plain balmy Sat night and certainly no risk for freezing
temps through the weekend. By stark contrast, lows Mon night
and Tue night will be dropping sharply and as low as the mid and
upper 20s.
The prospects for showers will be highest Sun and Sun night.
Currently POPs peak in the likely category, but later forecasts
may bump these up to categorical. There is a small risk for
showers during the day Sat, increasing Sat night. Mon should be
drying out with any risk for showers likely confined to the
morning hours with dry conditions continuing through Wed. The
warm temps and high dewpoints this weekend will be conducive for
late night and morning low stratus and fog with sea fog most
probable as the wind direction becomes favorable Sat afternoon
into Sun.
&&
.AVIATION /00Z THURSDAY THROUGH MONDAY/...
As of 23Z...The cold front is located along the coastal areas
from just south of KILM to just west of KMYR line and only
drifting slowly S-SE. Mainly light RA/SHRA around the area
concentrated near the coast and slowly shifting to the E.
MVFR/IFR cigs south of the front and around KFLO with VFR cigs
to the N. IFR cigs along the coast to gradually become MVFR
ceilings aft 08z-11z with MVFR ceilings inland terminals
becoming VFR roughly same time KLBT will drift in between VFR
and MVFR for a while then become VFR.
Winds will gust along the grand strand area along the frontal
boundary until FROPA as convergence is maximized in this area
as surface low passes offshore and then during the late morning
and afternoon with frontal boundary still in vicinity. As front
clears area late afternoon wind gusts will abate. models differ
on amount of moisture that linger across the terminals in the
way of clouds below 10Kft with the NAM profile more moist and
thus more pessimistic than GFS so VFR conditions might have a
solid deck of clouds around 5KFT instead of the BKN that is
forecast for the late morning into afternoon but shouldn`t
impact aviation concerns.
Extended Outlook...MVFR Thurs night, otherwise VFR. Possible showers
Sat/Sun/Sun.
&&
.MARINE...
NEAR TERM /THROUGH THURSDAY/...
As of 1000 PM Wednesday...The little wave of low pressure is now
offshore and the cold front has slipped through Georgetown, SC.
Northeasterly winds are now increasing along the coast north of
the Santee River. Small Craft Advisories are in effect north of
Little River Inlet where wind speeds should reach 20-25 kt
overnight with 6 foot seas away from shore. South of Little
River Inlet 15-20 kt sustained winds with gusts to 25 kt are
possible. The strongest winds are expected to develop after 2
AM.
Few changes are needed to the forecast with this update as
marine weather conditions deteriorate through the night.
Discussion from 300 PM follows...
Small Craft Advisory conditions will run from late this evening
through Thursday. Cold front making its way into the northern
waters early this afternoon with winds basically S-SW up to 10
kt through much of the waters ahead of it. As this front drops
south, winds will turn to the north behind it and increase in
intensity through late this evening. The SW flow ahead of it
should reach up near 15 kt just edging into the SC waters this
evening while the northerly surge behind the front will increase
up to 15 to 20 kt with higher gusts through tonight.
Seas running 2 to 3 ft will begin to feel the influence of the winds
with some 4 to 5 fters in the outer waters off SC but the main
increase will come from the northerly surge behind the front as it
makes its way south. Expect the greatest surge once the area of low
pressure shifts off the coast around or shortly after midnight into
Thurs morning. Expect seas to increase in northerly surge up to 5 to
7 ft across the NC waters. The higher seas will spread south but
should remain mainly off shore in SC with 3 to 5 ft seas with a few
6 fters possible in outer waters.
SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
As of 300 PM Wednesday...A changeable marine period, eventually an
improving one though. Thursday night, `Cautionary` headlines may
still be hoisted for 20 KT NE gusts and 5 ft seas offshore.
But into early Friday, a more welcoming marine trend, as a
northward lifting coastal warm front, removes a large portion of
the NE pressure gradient, easing winds, subsiding seas. The one
caveat is that visibilities may suffer Friday, as mist, drizzle,
lift northward over the waters Friday. Passage of this feature
signals the onset of return wind flow from the south late Friday
and air warming into the weekend. Waves this period will arrive
from the NE and E with dominant wave periods of 5-6 seconds,
and a diminishing NE chop late Thursday into early Friday.
LONG TERM /SATURDAY THROUGH MONDAY/...
As of 300 PM Wednesday...Ridge of high pressure will shift
S and E of the waters on Sat. A series of shortwave troughs
will help to deepen an upper trough across the middle of the
nation. The last of these significant shortwaves will lift out
across the Northeast states Sun night and Mon. This will bring a
slow moving cold front across the waters late Sun night and Mon
morning. The wind direction will be SW ahead of the front Sat
and Sun and then shift to W and NW behind the front, becoming N
Mon night. Wind speeds will be 10 to 15 kt, except up to 15 to
20 kt Sun night and Mon. Seas of 2 to 3 ft Sat will build to 3
to 4 ft for most of the rest of the weekend with perhaps some 5
ft seas across the outermost waters late Sun night into Mon
before subsiding to 2 to 3 ft Mon afternoon and 2 ft or less Mon
night.
The weather will a likelihood for showers Sun night into early
Mon and more in the way of scattered showers Sat night and Sun.
In addition, abnormally warm and sufficiently moist air will
coincide with a favorable wind direction this weekend, producing
an environment that will be conducive for sea fog development.
&&
.ILM WATCHES/WARNINGS/ADVISORIES...
SC...None.
NC...None.
MARINE...Small Craft Advisory until 6 PM EST Thursday for AMZ250-252.
&&
$$
SYNOPSIS...
NEAR TERM...TRA/RGZ
SHORT TERM...MJC
LONG TERM...RJD
AVIATION...MAC
MARINE...
Area Forecast Discussion
National Weather Service Marquette MI
809 PM EST Wed Dec 20 2017
.SHORT TERM...(This evening through Thursday)
Issued at 349 PM EST WED DEC 20 2017
Main fcst concern in the short term is light snow that will spread
across the area tonight/Thu. Water vapor imagery and RAP analysis
show a trof over the nw CONUS, low amplitude ridging out over the
Plains and a trof over se Canada/New England. Result is wnw flow
across the Upper Lakes this aftn. With 850mb temps around -15C over
Lake Superior, light LES continues off the lake, but with high pres
ridge nosing into the area and winds steadily diminishing, the LES
has been becoming more disorganized with time. Well to the w, fairly
strong isentropic ascent/waa shifting out over the Northern Plains
downstream of the trof over the nw CONUS is generating an area of
snow from ND into MN. Snowfall reports in ND indicate up to 2in of
snow has fallen there today.
Tonight...lingering light LES will continue until winds become
light/vrbl and shift offshore under passing sfc high pres ridging.
Attention then shifts to the snow spreading across ND/MN under
ongoing fairly strong isentropic ascent across the Northern Plains.
With the trof over the nw CONUS dropping s tonight instead of moving
e and only a weaker northern stream shortwave swinging across s
central Canada, this isentropic ascent will weaken as it shifts into
the Upper Lakes, mainly a result of the wind crossing the isobars
and an increasingly shallow angle compared to the more perpendicular
orientation currently over the Northern Plains. This should lead to
a broader/lighter snow spreading across Upper MI tonight thru Thu
morning before it fades away. Mixing ratios around 1.5g/kg are avbl
roughly in the 700-750mb layer, suggesting snowfall upwards of
3in/12hr, but given the broader/less focused ascent, 3in will be the
outlier high end potential amount per 12hr. So, at best, probably
looking at a 1-2/12hr snowfall into Thu morning. Light wind regime
with land breezes developing should keep any lake enhanced snow
bands out over the Great Lakes. If any location might be affected,
it would be the Keweenaw as convergence zone over the lake is fcst
to be in that vcnty. With the land breeze development, most high res
models show a mesolow developing over s central Lake Superior
somewhere btwn Stannard Rock and Pictured Rocks. Will be a potential
feature to monitor late tonight into Thu. If a mesolow does develop,
it would probably end up impacting part of Marquette/Baraga Counties
with locally mdt/hvy snowfall Thu aftn as gradient winds become e to
ne during the day, pushing the feature onshore while it weakens.
Otherwise, during the day Thu, upper jet streak forcing may lead to
another somewhat organized band of -sn spreading from southern MN
across at least s central Upper MI. As with tonight, shouldn`t
produce anymore than 1-2in. Lake enhancement off Lake MI could be a
concern, but most models indicate troffing/convergence zone off
Upper MI more toward the center of Lake MI with low-level ne flow on
the nw side of the trof. At this point, will plan for any lake
enhancement to stay offshore, but locations from Menominee ne to the
Garden Peninsula could easily be affected by heavier snow if the
trof/convergence zone drifts more to the nw than currently expected.
.LONG TERM...(Thursday night through Wednesday)
Issued at 308 PM EST WED DEC 20 2017
...Heavy lake effect possible Sat night into Sunday on Keweenaw then
bitter cold still on track for Christmas...
Late Thursday afternoon into Thursday evening well to the north of
1010mb sfc low over mid Mississippi river valley, isentropic lift at
285 to 295K sfcs (750mb-600mb) will hang around over south central
Upper Michigan. Also could see northeast flow lake enhancement
toward southern Menominee county with delta t/s around 15C.
Localized convergence will dictate where heaviest lake enhanced snow
occurs. Tough to pin down at this time though as high res models are
showing varying solutions with heavier snow occurring possibly as
far east as southern Delta county into southern Schoolcraft county
or maybe just getting hung up on the Door Peninsula. Will have to
keep things broad brushed at this point with pops/qpf/snow. Later
forecasts will provide appropriate fine tuning.
For the rest of Thursday night could see light snow (less than 1
inch of accumulation) linger over far eastern forecast area as weak
h85 low lifts across and there remains at least weak isentropic
ascent. Main focus for widespread snow will stay south over
Wisconsin and lower Michigan where stronger H8-H6 frontogenesis
resides along sharper H85-H7 baroclinic zone. Possible that as
deeper moisture departs late Thursday night could see any light snow
end as freezing drizzle. Farther north, light northeast winds
through 5kft AGL and just enough over-water instability (delta t/s
14c) may allow for light lake effect snow showers over higher
terrain of north central later Thursday night into Friday morning
before winds become southeast on Friday afternoon. Even as the lake
effect ends Friday afternoon, there could be patchy light snow or
flurries near Lk Superior and east ahead of a cold front that
crosses Friday evening. Behind the cold front later Friday night
into Saturday expect light lake effect to develop for northwest flow
areas. Inversions look low at around 5kft AGL and there is no
additional moisture or lift so looks like light lake effect snow
accums only helped out by DGZ occupying most of convective layer.
As upper level trough from Hudson Bay to central Conus continues to
expand later in the weekend, stronger sfc cold front is expected to
slide over Upper Great Lakes Saturday Night. Winds ahead of the
front will become more westerly so lake effect will become mainly a
Keweenaw Peninsula issue into Sunday. Sharp cyclonic convergence,
extreme over-water instability with delta t/s over 25C and lake
equilibrium levels over 10kft point to heavy lake effect snow. Some
of DGZ is still within convective layer to start and there is also
deep moisture to H7 as the front moves through. Eventually DGZ is
eliminated as it becomes colder through day on Sunday. Could be
period of efficient snow to start with higher SLRs but then it will
gradually transition to smaller snowflakes that will be very
efficient in reducing visibility but not be as good at producing
heavy snow accums (lower SLRs). Winds start out Sunday 20-25 kts but
then diminish to 15-20 kts which will help residence time of any
stronger bands and should restrict real severe visibility
reductions. Overall it is looking pretty likely that as long as the
wind directions stay westerly, we will need a headline across the
Keweenaw Peninsula some point Saturday night into Sunday. Main thing
to pin down will be where strongest convergence sets up (current
indications is somewhere from Calumet to Copper Harbor). Plenty of
time to fine tune those details.
Another clipper is forecast to come through Christmas Eve night into
Christmas Day as upper level trough further deepens and very cold
air (H85 temps -25C to -29C) charges into the region. Depending on
track the clipper takes, heavier lake effect on the Keweenaw may end
Sunday afternoon and evening or just re-orient to other snow belts
near Lk Superior. Given how cold it is with some sort of even weak
synoptic forcing moving through, lake effect could become squirrely
and feisty. For those who want to dig in a little deeper, reference
Christmas Eve through Christmas Night 2004.
Upper trough with minimum height contour at H5 of 491dam vcnty of
Hudson Bay and associated cold air with h85 temps down to -28c only
slowly relaxes through middle of next week. Behind the clipper early
in the week expect lake induced trough to keep lake effect mainly
over northwest to west snow belts near Lk Superior. Due to no DGZ in
convective layer leading to very small snowflakes, the lake effect
will be very good at lowering visibility but not as efficient at
producing big snow accumulations. Expect temps in single digits to
drop toward or below zero on Christmas Day, especially west half.
Inland areas could see min temps Tue morning 10 to 20 below zero.
Even though strongest winds will be near Lk Superior, wind chills
many areas could drop to at least 10 to 20 below zero and maybe even
toward 30 below west vcnty of Ironwood with stronger winds and lower
temps. That fits typical climo too. With more of a sfc ridge sliding
in Tuesday night, temps away from warming off Lk Superior could be
very cold, possibly lower than 20 below zero where winds go calm and
skies clear out. Beyond that, could be a widespread snow mid to late
next week, but most of models keep main focus farther south and east
on lead side of deep large scale trough.
&&
.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 759 PM EST WED DEC 20 2017
Lake effect snow showers persist off the Lake Superior shoreline;
however, these are expected to remain offshore and diminish in
coverage. Low CIGs will persist with bases around MVFR conds for
all TAF sites. Next system arrives later tonight with a return to
snow showers. VSBYs will likely diminish to around 3sm by 5Z then
further diminish in the stronger snow showers to 1sm arnd 7-12Z.
Expect a period of IFR at all terminals Thur morning. &&
.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 349 PM EST WED DEC 20 2017
Period of light winds tonight under passing sfc high pres ridge will
give way to somewhat stronger ne winds Thu into Thu evening (15-
25kt), mainly across w and central Lake Superior. Winds should then
be mostly under 20kt into Fri. Increasingly colder arctic air
flowing into the Upper Lakes this weekend and especially early next
week will result in stronger winds with gales at times. Right now,
best chance of gales looks to be on Mon. Very cold air mass will
also result in freezing spray with periods of heavy freezing spray.
&&
.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
None.
Lake Superior...
None.
Lake Michigan...
None.
&&
$$
SHORT TERM...Rolfson
LONG TERM...JLA
AVIATION...Beachler
MARINE...Rolfson