Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/24/17
Area Forecast Discussion
National Weather Service Quad Cities IA IL
854 PM CST Sat Dec 23 2017
Issued at 849 PM CST Sat Dec 23 2017
So far tonight, the 00Z data set continues to support the advisory
county placement. There is some short term mesoscale data / HRRR
that suggests amounts may need to be increased a bit more in the
north 1/2, with a more widespread 1.5 - 2.5 inch snow possible.
However, some of that area already saw a snow earlier this month,
and the "first" snow argument has less impact there. In any case,
snow is now falling in far southwest Iowa, and will spread east
overnight over the southern 1/3 of Iowa, and spread slowly
northeast towards morning. Most of the accumulations in Cedar
Rapids, Iowa City, Quad Cities will be in the 5 AM to Noon time
frame. A White Christmas is growing to near 100% for locations in
all but our far northwestern counties!
Otherwise, I have lowered temperatures for tonight, given the
rapid drop that occurred around sunset. Some locations in the
northwest dipped to the single digits prior to clouds spreading in
holding temperatures more steady.
Issued at 314 PM CST Sat Dec 23 2017
Shortwave diving through the central Rockies this afternoon will
shift east across the Mid-Mississippi Valley on Sunday, as it
begins to merge or phase with northern stream energy within
developing broad troughing. This system will spread light
accumulating snow across much of the cwa late tonight and Sunday
leading to a white Christmas for many.
.SHORT TERM...(This Evening through Sunday)
ISSUED AT 314 PM CST Sat Dec 23 2017
Model guidance in good agreement on developing a band of snow
late this evening and overnight across the southern cwa. This
snow will progress northward to near I-80 corridor around 12z,
then continue northward over much of rest of cwa through mid
morning as system pivots. Models have generally trended a bit
further north with snowfall, but still overall agree on highest
amounts occurring over southern cwa where deeper saturation and
bouts of lift into dendritic growth zone (DGZ) depicted.
Further north to I-80 and especially northward bulk of omega
look remain just above the DGZ promoting lighter amounts. Snow
ratios are shown between 15:1 and 20:1 and with models generally
averaging around 0.1 inch to 0.2 inch for liquid totals leads
to roughly 2-4 inch storm total accumulations across the southern
cwa, with amounts tapering off north of I-80 to less than an inch
across far northeast Iowa. The snow should be light/fluffy and
easy to remove. Meanwhile, winds will be fairly light as well
thus not expecting any blowing/drifting issues. These amounts are
on the lower end of advisory criteria but being the first
widespread accumulating snowfall and roads busy with holiday
travelers a winter weather advisory looks perfect, and in fact
so much so that I have expanded it through the I-80 corridor to
account for busy interstate travel despite amounts being fairly
light. The snow will likely taper off to flurries and end from west
to east mid morning through mid afternoon.
.LONG TERM...(Sunday Night through Saturday)
ISSUED AT 314 PM CST Sat Dec 23 2017
Long term forecast is focused on cold temperatures heading into the
late week. The snowpack from the snow forecast tomorrow will have
a huge impact into next week. At this time, changes to represent
the effects of the snowpack have not been made. Overall few
changes to the long term pattern have been noted with this run of
the guidance. Overall, strong high pressure will limit impactful
precipitation. Guidance still forecasting a 1040+ hPa high across
the Upper Great Plains and Mississippi River Valley.
Late in the period, Thursday, looks to be the next chance for
precipitation across the area. There are certainly differences in
the timing and amount of QPF between guidance. That said, this is
most likely the next chance the area could see snow. At this time,
chance pops are in the forecast.
.AVIATION...(For the 00Z TAFS through 00Z Sunday Evening)
ISSUED AT 526 PM CST Sat Dec 23 2017
The first snow event this cool season will impact the eastern Iowa
and northwest Illinois airports from early Sunday morning, through
early Sunday afternoon. The snow will be fluffy, and easily moved,
and winds will be generally light northwest, thus removal from
runways should be efficient. Visibilities will drop to MVFR in the
08Z to 12Z window, and then to IFR through the morning as snow
falls with lower MVFR to IFR cigs. By afternoon, snow will sweep
east, ending in local area airports. A period of very light snow
and MVFR cigs may linger through late afternoon, followed by
increasing northwest winds and VFR conditions.
IA...Winter Weather Advisory from midnight tonight to noon CST Sunday
for Cedar-Des Moines-Henry IA-Iowa-Jefferson-Johnson-Keokuk-
Winter Weather Advisory from 3 AM to 3 PM CST Sunday for Clinton-
IL...Winter Weather Advisory from midnight tonight to noon CST Sunday
Winter Weather Advisory from 3 AM to 3 PM CST Sunday for Bureau-
Henderson-Henry IL-McDonough-Mercer-Putnam-Rock Island-
MO...Winter Weather Advisory from midnight tonight to noon CST Sunday
Area Forecast Discussion
National Weather Service Wilmington OH
1019 PM EST Sat Dec 23 2017
An upper level disturbance moving through the Ohio Valley will
bring wintery precipitation to the region for Christmas Eve.
High pressure will build into the region on Christmas Day,
resulting in dry and cold conditions. Another upper level
disturbance will move across the area Tuesday bringing another
chance of snow. For the remainder of the week temperatures look
to remain below normal.
.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
Clouds have started to break from the northwest with widespread
mid and high clouds overhead. The main point of interest for
this update was the chance of snow on Christmas Eve. Latest 00z
run of the NAM continues to have a little bit more robust QPF
with an approaching shortwave which will likely bring a chance
of snow to the area starting Sunday afternoon into evening. The
initial band of precipitation will move into our northwestern
zones starting late Sunday morning. The latest runs of the ARW/
NMM/ RAP and NAM nest have now displaced the band of
precipitation slightly further north. Later Sunday afternoon
(around 21z) the piece of PV will approach from the west. The PV
looks to start to curl which would favor a FGEN band on the
western side of the PV. Latest NAM shows the FGEN response when
looking at the 700/ 600 mb Fn vectors. High res models show this
as a band of snow approaching from the west. What this will
mean is that most of the area will likely remain snow and
precipitation free through Sunday morning into early afternoon
(except our northwestern counties).
This second batch of snow in association with the PV will likely
cross the area starting late afternoon and thermal profiles
support mostly all snow with this (thanks to shortwave cooling).
GFS max wet bulb temperatures also quickly fall off as the
shortwave and PV move across the area. Towards our far southeastern
zones p-type will start as a rain/snow mix before quickly
transitioning over. Given the above have trended snowfall totals
across our northwestern counties down slightly while raising
snowfall totals slightly across the central zones. Have left the
WSW as is while also increasing wording in SPS.
Have also decided to do a non- diurnal temperature trace. NAM
forecast soundings also support some gusty winds as the
shortwave moves overhead (the 1.5 PV surface down to ~600 MB
across the south). Due to this have used wind gusts from
momentum transfer. New zones already out.
Prev Discussion ->
Precipitation has shifted off to our east late this afternoon
with even a few breaks developing in the clouds across our
northwest. As we continue to see some drying work into the area,
we could see at a few more breaks develop through the evening
hours, but for the most part, will continue with a fair amount
of clouds for tonight. Overnight lows with be in the mid to
.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
Mid level short wave energy will approach from the west during
the day on Sunday and then push east across our area Sunday
night. An axis of better forcing will work in across our far
north through early afternoon but there is some uncertainty
with exactly where this will line up. Right now, it looks like
it will be along or just north of our far northern CWA. As a
result, expect pcpn to develop into northwest portions of our fa
through late morning and then continue into the afternoon.
Farther south, the better chance of pcpn will likely hold off
until later in the afternoon and then into Sunday night as the
short wave and trough axis moves across the region. Weak
southerly low level flow will develop through early afternoon
ahead of the 850 mb trough. This will allow for some warmer air
to work in across the south and east. Highs on Sunday will range
from the upper 20s across our northwest to the mid and possibly
upper 30s across the south/east. This will allow for some of
the pcpn to possibly start off as a rain/snow mix or even all
rain across parts of of the south and east. Colder air will move
in on the back side of the trough, allowing for the pcpn to
change over to all snow by by Sunday evening.
Expect snowfall amounts across our northwest generally in the 1
to 3 inch range. Suppose a few four inch amounts may be
possible across our extreme northwest, depending on exactly
where the initial band of snow sets up through Sunday
afternoon. While we may end up marginal on advisory criteria,
went ahead and issued an advisory for our northwest with it
being Christmas Eve. Farther to the south, snow totals through
Sunday afternoon will depend surface temps and how much rain
mixes in with the snow and then how fast we change over to all
snow heading into Sunday evening. For now, generally have 1 to
locally 2 inches elsewhere north of the Ohio river through
Sunday night with generally an inch or less along and south of
the river. With still some uncertainty in the accumulations and
the fact that most of the accumulating snow would be heading
into Sunday evening, have opted to hold off on an advisory
elsewhere across our fa. Will continue to mention the
possibility of accumulating snow in an SPS.
.LONG TERM /MONDAY THROUGH SATURDAY/...
Fcst centers around arctic air through the week with some
uncertainty surrounding late week snow event.
Eastern two thirds of country will be in the grasp of large trough
with arctic air settling in through the upcoming week.
Thickness...850 temps and guidance all pointing to coldest air so
far in the season with lasting power through the upcoming new years
weekend. Will stay close to guidance through period. Any snow on
ground and clearing could lead to colder lows. Clouds could be
stubborn with lake effect and flurries possible much of the week.
Some hint at a quick light snow shower event Tuesday but will keep
low chance pops for now.
Biggest uncertainty lies in outcome of shortwave moving across
southern tier into northeast Friday and Saturday. Models have mixed
solutions but enough of a concern to certainly keep the low pops for
snow in for now.
.AVIATION /03Z SUNDAY THROUGH THURSDAY/...
All TAF sites MVFR this evening as the upper level disturbance
pushes east. Skies are already starting to clear across our far
northwestern counties as drier air temporarily works into the
area. This means all TAF sites will go VFR by 5z tonight.
Another upper level disturbance will quickly push east making
the VFR cigs transient with MVFR cigs quickly moving in from
the west. By late Christmas Eve morning all taf sites will
already be MVFR. A band of snow in association with weak
midlevel frontogenesis will push east across our far northern
zones, but the TAFs should remain dry for this first round of
snow. Later Sunday afternoon into evening forecast soundings
completely saturate as the core of the upper level disturbance
moves overhead. As this occurs snow will spread over the region
with visibilities potentially falling to LIFR. For now have only
trended vsbys down as timing between high res models remains
OUTLOOK...MVFR/IFR ceilings and visibilities likely into Sunday
night. MVFR ceilings possible Monday night into Tuesday.
OH...Winter Weather Advisory from 9 AM Sunday to 1 AM EST Monday
IN...Winter Weather Advisory from 9 AM Sunday to 1 AM EST Monday
Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
927 PM CST Sat Dec 23 2017
Issued at 926 PM CST Sat Dec 23 2017
Regional radar and surface observations are showing that a band of
light snow extended across Nebraska into northwest Missouri at mid
evening. Evening upper air analysis showed considerable low level
dry air ahead of this band of snow over Missouri and Illinois. The
RAP does show enough mid level ascent to help generate snowfall
rates that will saturate the low levels from northwest to
southeast overnight into early Sunday. Going timing of snowfall
moving into the area and snowfall amounts through tomorrow still
look good at this point. Made only some minor short term
adjustments the next few hours to temperature and winds based on
.SHORT TERM... (Through Late Sunday Afternoon)
Issued at 244 PM CST Sat Dec 23 2017
The next shortwave upstream is digging into southeast Idaho and
western Wyoming at this time. This wave will bring the area another
round of light snow tonight and Sunday morning. Cold air now in
place over the region should ensure that all precip will fall as
snow tonight and Sunday. Short range guidance shows an area of
decent low level warm advection ahead of the wave as the 850mb wind
turns to the south-southwest. This will be coincident with
increasing low level relative humidity across northern Missouri
after 06Z. Initial precip across our northern zones may be fairly
light, but should intensify as the boundary layer saturates. Think
most of the precip will stay in the current advisory area across our
northern zones until after 10-12Z but it will eventually spread
southeast as the shortwave swings through the area on Sunday. Snow
should end from northwest to southeast through the morning into
early afternoon Sunday. Moderate to strong low level frontogenesis
in the vicinity of the front could produce some moderate snow bands,
but the bands should be transient and not produce any prolonged
periods of heavier snow. 2-4 inch amounts in the advisory
area still look very reasonable with lesser amounts to the south and
southeast. Therefore no changes are anticipated in either the
area or the timing of the advisory at this time. Temperatures
tonight and Sunday are definitely colder than the past several
days with lows dipping into the teens and 20s tonight and then
struggling to make it above 30 in most locations on Sunday.
.LONG TERM... (Sunday Night through Next Saturday)
Issued at 244 PM CST Sat Dec 23 2017
The synoptic pattern at the start of the long term period will
feature cyclonic, northwest flow aloft. This regime will persist
throughout the entirety of the long term period, keeping below
normal temperatures in place along with a few chances for mainly
All snow will have exited the region Sunday night, leaving
dry conditions in its wake. Given the expected light snowfall
(especially across northeast MO/western IL), temperatures will
likely quickly fall after sunset. Winds staying up just enough may
mitigate the radiational cooling conditions somewhat, but still
think lows in the teens seem quite reasonable, with even lower
values likely in areas that receive a couple inches of snowfall.
Monday (Christmas Day) looks dry as surface ridging slides across
the area. It will be a chilly start to the day, but lower
tropospheric flow will turn to the WSW which should allow
temperatures to recover into the low/mid 30s.
Predictability begins to decline by Monday night into Tuesday.
As is typical in swift northwest flow regimes, guidance is
struggling with the timing/strength/placement of PV anomalies moving
southeast through the flow. These differences show as early as
Monday night with the Euro quicker with bringing a band of light
snow than the GFS/NAM. Will lean toward the latter solution and keep
slight chance pops in the forecast Tuesday morning. This setup is
notorious for producing localized areas of snowfall that makes it to
the ground in a narrow band beneath the strongest f-gen forcing,
thus higher pops will likely be needed as we dial in where this
narrow band may set up. Either way, as it looks now, this looks to
be a very minor snow event.
The main story by Tuesday into Wednesday will be the cold
temperatures (likely the coldest of the season thus far). With
strong cold air advection expected on the eastern periphery of a
1040+ mb surface ridge, temperatures likely won`t make it out of the
teens for highs over northeast MO and western IL, with 20s expected
elsewhere. Tuesday night will be the coldest night of the year thus
far, with lows in the single digits. It is possible some of
the typically cool areas that still have snowfall on the ground may
dip below zero early Wednesday morning.
Temperatures will remain generally in the 20s for highs and in the
teens for lows through the rest of next week. We continue to watch
for the potential for another wintry system to affect the area on
Thursday. Guidance has flipped from 24 hours ago, with the ECMWF
solution now suggesting a stronger system than the GFS. It appears
the crux in this forecast is with how deep the PV anomaly digs,
affecting how much Gulf of Mexico moisture is able to be lifted
northward ahead of this system. Given this system is still way
offshore, we will likely see these discrepancies continue for the
next day or two. For now, have kept chance pops in the forecast for
Thursday which keeps good consistency with the previous forecast.
.AVIATION... (For the 00z TAFs through 00z Sunday Evening)
Issued at 542 PM CST Sat Dec 23 2017
VFR ceilings and conditions will persist through 07z over the
region. A storm system currently over the Great Plains will push
into Missouri late tonight. Light snow will begin falling over
parts of north central Missouri between 7-9z with ceilings falling
to MVFR. There will be periods of IFR at both UIN and COU between
12z and 14z due to snow. MVFR ceilings will push southward and
impact the metro terminals between 13-15z with a brief period of
IFR ceilings and visibilities possible. Low ceilings will begin to
lift to MVFR then over north central Missouri by late Sunday
SPECIFICS FOR KSTL: VFR ceilings and conditions will persist
through early Sunday morning. Light snow will impact the terminal
around 12z bringing in MVFR ceilings with period of IFR ceilings
possible from 13-15z over the terminal. The low ceilings will
begin to lift to VFR after 00z as the low clouds push off to the
MO...Winter Weather Advisory until noon CST Sunday for Knox MO-Lewis
MO-Marion MO-Monroe MO-Ralls MO-Shelby MO.
IL...Winter Weather Advisory until noon CST Sunday for Adams IL-Brown
Area Forecast Discussion
National Weather Service Melbourne FL
841 PM EST Sat Dec 23 2017
Fog is the only significant forecast problem for the overnight as
dew points will continue to be abnormally high for this time of the
year. A high pressure ridge sagging into south Florida will produce
a light southwest low level wind flow, except light/variable in the
south near the ridge axis. Considerable cirrus clouds will move over
the area through the night, but it appears that there will be less
stratocumulus than last night. Boundary layer flow may pick up
close to 10 mph in the north, ahead of a cold front moving slowly
across the Florida panhandle. This may inhibit radiational fog
development in the north, but advection of fog from the eastern Gulf
of Mexico to inland sections cannot be ruled out.
MOS suggests fog over the interior while mesoscale models have
various solutions, ranging from widespread coverage to little/none.
Our current forecast mentions areas of dense fog over the interior
and patchy fog in the coastal counties. Confidence is not high
enough to make any changes at this time.
There will be a chance for IFR-LIFR conditions in fog overnight,
particularly at the interior terminals. Our forecast has lowered
visibilities there, but MOS guidance hasn`t been showing
accompanying ceilings. This usually suggests patchy IFR-LIFR
coverage. The HRRR model has only been indicating patchy fog, so
confidence is not high enough to pick which interior terminals will
be most significantly affected, though KLEE should have the highest
chance for IFR-LIFR.
Tonight-Sunday...Boating conditions look good as the axis of
Atlantic high pressure ridge extends across our southern waters.
This will provide a light south wind flow in the south and southwest
winds around 10 knots in the north. Seas will be 2 feet or less
along the coast and 2-3 feet offshore.
Area Forecast Discussion
National Weather Service Pueblo CO
316 PM MST Sat Dec 23 2017
.SHORT TERM...(This evening through Sunday)
Issued at 316 PM MST Sat Dec 23 2017
The main forecast concern is the winds for tonight. Snow is ongoing
in the central mtns and will continue through the evening hours with
some additions accumulations. However, later tonight as an upper
level disturbance moves out of the area, snow will decrease or end
in the central mtns, with maybe just some light showers lingering. A
140 kt jet will be over the area tonight. There are already strong
winds being reported in the central mtns, with gusts of 60+ kts at
KMYP, and gusts near 50 kt at KVTP. MOS guidance for KMYP has very
strong winds with most values being between 50 and 60 kts, while the
MAV guidance has 70+ kts. The HRRR is showing potential gusts of 65
kts. Have decided to go with a High Wind Warning thru late tonight
for zones 58 and 60. Will go with strong gusts over all the mtn
areas tonight, but will keep gusts under 75 mph in those areas in
the forecast. A cold front will make its way south through the
southeast plains tonight. At this time, the HRRR is showing it
moving thru KCOS between 03Z and 04Z, and then through KPUB around
05Z. MOS guidance has been showing some strong winds behind the
cold front and we currently have 40-45 mph winds in the forecast.
However, the latest HRRR graphics are coming in with winds that are
not quite as strong. There will still be the potential for some
light snow over the Palmer Divide and northern Teller counties after
the front moves thru, but nothing significant is expected and it
should be fairly short lived.
Generally dry weather is expected on Sun, although some snow showers
will likely develop late in the day over the central CO mtns ahead
of the next weather system. Temps will be below average over the
southeast plains on Sun, and should be around or slightly above
average in the San Luis and Upper Arkansas River Valleys.
.LONG TERM...(Sunday night through Saturday)
Issued at 316 PM MST Sat Dec 23 2017
Sunday night and Monday...NAM, EC and GFS models agree on an upper
disturbance crossing the region Sunday and Monday, with strong west
to northwest flow aloft. Snow will develop along the Continental
Divide Sun evening and spill over to the remainder of the higher
terrain overnight, with the focus area being the central mts. The
central mts may see new snow accumulations of up to 8 inches through
Monday evening, while the rest of the peaks may receive 1 to 3
inches. Since the central mts are currently in a winter highlight,
will not hoist another to avoid confusion. Look for min temps
Christmas Eve to fall into the teens to lower 20s for most areas,
then the brisk nw flow aloft will help boost aftn readings on
Christmas into the mid 40s to lower 50s for the plains, and 40s for
the high valleys. A cold front is forecast to drop south across the
Palmer Divide Monday aftn, but the true effects of the front are
not anticipated until the overnight hours.
Tuesday through Saturday...Extended models agree on not a lot of
change overall in the weather pattern for the rest of the week.
Brisk w-nw flow aloft continues across the region, with an
occasional isolated threat of some showers for the central mts, most
notably on Wed. Otherwise, predominately dry. Tuesday will be the
coldest day due to the cold front from the previous day, and due to
strong high pressure at the surface pushing back into the eastern
plains and offsetting the general wind flow. On Tue, forecast maximum
temps will range from the mid 20s to mid 30s across much of the
plains, 40s for the high valleys, and perhaps 40s right along the
eastern edges of the Wets, Sangres and Rampart Range where the
downslope w-nw winds will hold at bay the encroaching easterly llvl
flow. Where that exact boundary sets up is going to be a challenge.
For Wed through Sat, expect highs in the upper 30s to upper 40s for
most areas. Moore
.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 316 PM MST Sat Dec 23 2017
VFR conditions are expected the next 24 hrs at KCOS, KPUB and KALS.
A cold front will move through southeast Colorado tonight, KCOS will
likely see frontal passage and gusty north winds between about 03Z
and 04Z, and KPUB should see it move thru around 05Z. Some gusty
westerly winds will be found early this evening at KALS, but these
are expected to decrease later in the evening.
High Wind Warning until 4 AM MST Sunday for COZ058-060.
Winter Weather Advisory until 11 PM MST this evening for COZ058-
Area Forecast Discussion
National Weather Service Blacksburg VA
838 PM EST Sat Dec 23 2017
Cooler air will filter into the region tonight behind a cool
front; however, a much more significant push of colder air is
expected later Sunday into Monday - accompanied by some light
snows in the mountains. Temperatures will fall to well below
normal by Christmas Day, and remain below normal for much of the
.NEAR TERM /THROUGH SUNDAY/...
As of 835 PM EST Saturday...
Cool front and associated line of showers has now cleared the
extreme southeastern portion of the Blacksburg forecast area, so
have made some minor adjustments to remove precipitation threat
across the southeast. Increasing downsloping winds will help
erode cloud cover east of the Blue Ridge for the rest of the
night. West of the Blue Ridge, upsloping winds should keep a
good amount of cloud cover over the mountains, along with a
threat of some light rain/snow showers or sprinkles/flurries
over the western flanks of the Appalachians - mainly in SE WV.
No other changes needed to the existing forecast at the present
As of 628 PM EST Saturday...
A narrow line of gusty showers has developed this afternoon
and is now entering the Virginia Piedmont, Southside into the
northern North Carolina Piedmont in association with primary
cold front. This feature actually trails northeastward to just
west of the D.C. metro area as of this writing, occasionally
exhibiting some enhanced reflectivity and brief downpours along
its full extent. Fortunately this feature is nearly through much
of the forecast area, but it will still continue to produce a
brief period of downpours as it advances east. Used a heavy dose
of the HRRR to re-shape/reduce PoPs areawide through 02z;
thereafter confining residual Chance-level PoPs to the western
upslope Appalachian areas in southeast West Virginia and far
southwest VA through overnight. Most areas are dry except for
these far western sections, and even there, QPF is not expected
to be substantial.
Largely due to strong isallobaric pressure rises in the wake of
the cold front - this is indicated in recent METAR remarks such
as at Roanoke and Bluefield - gusty winds have resulted along
the Blue Ridge/foothills into southeastern West Virginia. Gusts
have been at times as high as 40 mph, having been most recently
observed at Boone. Fortunately, this increase is short- lived as
well per recent forecast guidance, and so I`ve only boosted
winds/wind gusts through 02z using a blend of MAV-based MOS into
the current forecast. Northwest winds should then begin to
lighten into the overnight and begin to turn to a
northeasterly direction toward the early AM hrs.
Look for a gradual fall in temperatures and dewpoints from west
to east. Will need to watch far southwest Virginia and the
southern Blue Ridge in NC for potential fog but due to mixed
signals in guidance, I wasn`t confident enough to add into the
grids. It would likely be late tonight/overnight if it occurs
when winds project to be the lightest.
Previous discussion issued at 335 PM follows...
A cold front will move southeast tonight and stall along the North
Carolina coast Sunday. A secondary surge of cold air will push into
our region Sunday into Monday.
The timing of the front is close on both GFS and NAM this evening,
then the models slow down or try to stall the frontal boundary as it
reaches North Carolina tonight. The main batch of gusty showers
entering the piedmont this afternoon should exit our forecast area
this evening. 850MB winds immediately behind the front are around 40
knots from the northwest then drop off during the evening into
tonight. As been the trend in the models, the 6hr pressure rises are
robust in the +5 to + 10 MB range. This will result in gusty winds
continuing into this evening. Some clearing is expected east of the
Blue Ridge this evening and overnight. Clouds will linger on the
western side of the central Appalachians through the overnight. Any
upslope showers in the northwest will diminish as the moisture
becomes shallow. A light accumulation snow accumulation less than an
inch is possible in western Greenbrier county. Clouds may persist
in southern Virginia and northern North Carolina late tonight as the
surface and low level winds come around to the northeast in response
to low pressure to the south. Low temperatures tonight will range
from the mid 20s in northwest Greenbrier County in southeast West
Virginia to the lower 40s in the piedmont. The coastal low will lift
northeast away from the coast line in the Atlantic ocean. An upper
level trough will rotate east out of the Midwest Sunday. A second
surge of colder air will result in the chance of showers in the
west. High temperatures on Sunday will vary from the mid 30s in the
mountains to the mid 40s in the piedmont.
.SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/...
As of 335 PM EST Saturday...
A weak wedge over the area will continue to erode as low pressure
tracks northeast across the southern Ohio Valley Sunday evening.
However, erosion of the wedge will be across the mountains into
northern Virginia, leaving a bubble high over southwestern Virginia
and northwestern North Carolina foothills and piedmont. This bubble
high will keep areas east of the Blue Ridge dry through the evening.
After midnight, this bubble will pop as a cold front slides over the
mountains. Winds will become westerly very quickly, which will
continue to keep the east dry going into Christmas Day.
As for the mountains, low pressure will track northeast along the
front into the Ohio Valley from late Sunday afternoon into Christmas
morning. Even though the surface front will move across the
mountains by midnight, the surge of colder air into the region will
wait until the low moves over Lake Erie (early Christmas morning).
Evening temperatures will run in the mid 30s west of the Blue Ridge
to low 40 east. By sunrise, temperatures will be in the low to mid
20s west to low 30s east. There will be some light precipitation
accompanying the frontal passage Sunday night into Christmas morning.
The bulk of the moisture and heavier precipitation will stay north
with the surface low tracking along the front. With most locations
being above freezing Sunday evening, precipitation will stay as
rain, higher elevations above 3000 feet may see a change-over to
snow after sunset. As colder air filters in behind the front,
precipitation will change over to snow. As drier air moves in after
midnight, snow showers will mainly be confided to western slopes of
southeastern West Virginia into Christmas morning. The duration of
mountain snow showers will be limited to around 6 hours, while
western slopes could see snow for 12 hours. After all is said and
done, accumulations across most of SW VA/NW NC mountain valleys will
range from a dusting to half an inch. Higher elevations could see up
to an inch. Accumulations in the far west (Richlands-Bluefield-
Lewisburg) may see half to one inch of snow, while higher elevations
could see 1-3 inches of snow, particularly western Greenbrier.
Besides light snow showers and cold temperatures associated with
this cold front, strong and gusty winds will also be a concern.
Strong pressure rises behind the front along with a 40 kts low level
jet will bring near advisory level winds to the area Sunday night
into Monday. Guidance numbers along the counties bordering the Blue
Ridge support a low-end wind advisory. However, forecast soundings
showing a high inversion (above 3kft) and saturated low levels. If
winds were to reach advisory criteria, it looks to be a 2 to 4 hour
window between 4a-10a Christmas morning. Winds will begin to subside
through the afternoon as high pressure builds over the region. Since
this is a 3rd/4th period low confidence forecast, we will hold off
on any headlines with this forecast package. Also, with windy
conditions and cold temperatures, wind chills values will range from
the single digits to low teens across the mountain valleys to near
zero along higher elevations. East of the Blue Ridge, wind chill
values will range from 15F to 25F. Forecasted afternoon air
temperatures Christmas day will range from the upper 20s to low 30s
west and upper 30s to low 40s east.
High pressure is over the region Monday night with less wind but
colder temperatures. Overnight lows will run 15F-25F across the
mountains to the upper 20s in the east. Tuesday will seem warmer
with temperatures in the 30s west to 40s east. However, these
temperatures are 3F-6F cooler than normal.
.LONG TERM /TUESDAY THROUGH SATURDAY/...
As of 320 PM EST Saturday...
Both the GFS/ECM look similar Tuesday into Thursday. An area of
high pressure will track over the region Tuesday with a mid level
disturbance moving east in zonal flow. This disturbance will blanket
the area with clouds, keeping the area cool with temperatures
ranging from the upper 30s to upper 40s. On Wednesday, a mostly dry
back door cold front will send temperatures below normal which will
remain the trend going into the final weekend of the year. Behind
the front, high pressure will wedge south across the region, sending
temperatures into the 20s. Based on the consistent ECM, a southern
stream disturbance will move from the Mississippi Valley to the
southern Appalachian Mountains Thursday. This disturbance will
push moisture over the wedge with precipitation starting
Thursday night. As the northern and southern streams begin to
phase on Friday, a surface low is expected to form off the SE
coast, then track north along the Mid Atlantic coast and become
a strong noreaster off the New England coast. Temperatures start
out cold enough for precipitation to start out as snow. As the
disturbance slides to our south, warm air may edge into the
southern half of the CWA for a change over to rain/freezing
rain. It is possible that the area stays cold enough for this to
be an all snow event. Time will tell. The GFS has a different
scenario. The GFS has the wedge dipping down into the Carolinas
Thursday with any disturbances stay south along the Gulf coast.
The wedge is removed by a cold front Friday night into Saturday.
It is also worth noting that the GFS does not phase the
northern and southern stream at all next weekend. With such a
big change in the GFS, we kept with the constant ECM.
.AVIATION /02Z SUNDAY THROUGH THURSDAY/...
As of 641 PM EST Friday...
Narrow line of showers moving across the Piedmont is resulting
in short-lived MVFR visibility showers and gusty northwest winds
which at times have reached 35 kts. Gradual decrease in gusts
expected areawide through 02z but will remain sustained around
6-10 kts through the evening. After 04z, any sub-VFR enhanced
by northwest upslope is focused along the western Appalachians,
mainly affecting Bluefield and Lewisburg. Lingering showers,
mixing with wet snow at times, may also be possible at these
sites but confidence wasn`t high enough to include mention in
the TAFs. Anticipate brief window of VFR elsewhere after 04z.
Easterly flow becomes enhanced Christmas Eve as surface ridge
builds to our north. This will increase moisture and by most
model guidance an expectation for lowering ceilings is
indicated after 16z. This ahead of the next frontal system
coming out of the Plains/Ohio Valley very late in the 00z TAF
period. Strong cold front may introduce snow showers perhaps as
soon as 00z Sunday but little consistency in timing noted.
Therefore, will indicate VCSH late in the period to account for
timing uncertainty. Winds likely easterly initially around 4-8
kts Christmas Eve, but will veer to southwest late at Bluefield.
Medium confidence on ceilings, visibilities and winds during
the taf period.
The arrival of an even colder airmass Sunday night into Monday
results in yet another round of gusty winds and MVFR to IFR
flight restrictions. Surface high pressure building in from the
west should support a return to VFR weather for all mountain
terminal forecast sites Tuesday into Wednesday. Sub-VFR
conditions are possible across the region Thursday into Friday
with potential for wintry precipitation.