Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/09/17
AREA FORECAST DISCUSSION
National Weather Service Jackson KY
954 PM EST Fri Dec 8 2017
.UPDATE...
Issued at 954 PM EST FRI DEC 8 2017
The latest observing and model trends continue to point to little
to no snow across the southeast tonight. The outlier has been the
HRRR and that model is now showing a drier trend in the latest
runs. This continues to fall more in line with the radar and
satellite trends as well as the latest downstream observations.
That said, did keep a slight in the fringe higher terrain spots
otherwise think most areas will remain dry. Another challenge has
been the mid to high level clouds that continue stream in from the
southwest and the impact on the temperature curve. Overall have
kept with the previously modified low temperature grid, but this
may have to be modified further based on the cloud cover trends.
The latest grids have been finalized and sent. Also updates were
made to HWO and ZFP to reflect latest forecast thinking.
UPDATE Issued at 642 PM EST FRI DEC 8 2017
The evening surface analysis is showing a small influence from
surface high nosing northward. Otherwise we remain stuck between
two systems this evening. This keeps us mostly driven by the
strong jet energy aloft and disturbance to the east that continue
to render mid to upper level clouds across eastern Kentucky. This
being to case did increase the overall cloud coverage for portions
of the night. There remains some concern on weaker moisture
making it into the far southeast tonight, however, the short term
models are overdone based on current trends. These models have
been most robust with the QPF and given this confidence remains
low this evening for any snow. Therefore, did lower POPs and QPF
in the southeast and this subsequently leads to lower snowfall
potential. The cloud cover also leads to less confidence in the
low temperatures tonight and did lean slightly higher based on
that idea. Otherwise loaded the grids with latest obs and trends.
&&
.SHORT TERM...(This evening through Saturday night)
Issued at 247 PM EST FRI DEC 8 2017
We have two weather systems we are monitoring this afternoon. The
first one is pushing northeast across the southeastern states with
a large plume of moisture extending across the southern
Appalachians. The second system is diving southeastward across the
far western great lakes. Presently, eastern Kentucky is sandwiched
between these two systems, with nothing more than a bit of mid to
high level cloud cover. This will change for a few folks tonight
as moisture associated with the system to our south brushes the
Kentucky counties bordering Virginia. RAP/HRRR have been wavering
around with the northward extent of precipitation over the past 3
or 4 runs, but its looking more likely that some light snow will
push into our counties bordering Virginia late this evening or
overnight. RAP/HRRR/GFS all indicate about 0.05 to 0.10 inch of
liquid equivalent tonight, which would support around an inch of
snowfall with a snow ratio near 15:1. The highest totals will
likely reside in the highest terrain, with drier air eating away
at totals at lower elevations. For all other locations, expect
skies to gradually clear overnight. The clearing skies and
expected light winds will allow for temperatures to plummet
tonight. Lows will fall well into the teens in many areas. The
exception will be in the far southeast where precipitation and
clouds keep temperatures in the 20s.
Any snowfall will end around daybreak Saturday with a mostly sunny
start expected. However, the second system will roll into eastern
Kentucky from the northwest by midday or early afternoon. This
will bring an increase in cloud cover and increased chances for
snow showers. Low level lapse rates will steepen up quite a bit
through the early evening hours, allowing for a few intense snow
showers that could lay down a quick coating of snow. This is
despite highs expected to reach into the mid 30s. We will likely
see temperature falls within the snow showers. However, given the
air temperatures near or above freezing, travel conditions should
not be all that impacted. Expected liquid equivalent is only going
to be a few hundredths of an inch for most, so this won`t support
that much snow. Perhaps up to a half inch in favored westerly
upslope regions. A few snow showers or flurries may linger into
Saturday evening before coming to an end. Most of the moisture
with this system will fall in the dendritic growth zone, so we
could see some decent dendrites with the snow on Saturday. This
could aid in some instances of reduced visibilities, impacting
travel in the heavier snow showers.
Temperatures Saturday night will drop back into the teens again
with another strong push of cold air moving in behind the clipper
system.
.LONG TERM...(Sunday through Friday)
Issued at 410 PM EST FRI DEC 8 2017
The long term period is expected to begin with a western Conus
ridge and an eastern Conus/eastern North American trough. The axis
of this trough is expected to have moved east of the area by the
start of the period or shortly thereafter. At that point, surface
high pressure is expected to be centered over the Southern Plains
and southeastern states. The clipper system/shortwave that is
expected to affect the region in the near term will have moved
east of the area with another clipper system working through the
Great Lakes at that point.
Height rises are generally anticipated from Sunday into Monday
with the eastern Conus trough becoming more broad in general and
the airmass modifying. In fact, 850 mb temperature are expected to
recover from near -9C or -10C to start the period to around 3C or
so by Monday evening with southerly flow between high pressure
departing to the south and east and the next clipper system
dropping into the MS Valley on Monday.
Like the previous clipper system, the one for Monday night through
Tuesday is again expected to track north of the area a track
typical of minimal wintry precipitation. The cold front
associated with this system should move across the area on Monday
night with a few snow showers and flurries with additional
coverage of snow showers possible on west to northwest flow and
increased instability due to solar insolation on Tuesday. At this
point, isolated to scattered coverage of this is expected with any
accumulations mainly above 2000 feet and limited to an inch or
less as vertical motions should peak below the dendritic growth
zone with limited moisture. The coldest of the airmass should move
across the area on Tuesday night with 850 mb temperatures
bottoming out near -15C.
The period from late Tuesday night through Wednesday night appears
to be in between clipper systems at this time, with cold high
temperatures and a fair amount of mid and high level clouds.
There is uncertainty regarding the track and timing of the last
clipper system of the extended period which should arrive late
Thursday or Thursday night with light precipitation chances
lingering into Friday. What falls, if anything with this system
should primarily be snow.
After a cold start, highs should average 10 to 15 degrees below
normal on Sunday. Clouds and some mixiness due to a moderate
pressure gradient should keep temperatures from bottoming out on
Sunday night. Highs should moderate to near normal levels for
Monday. As colder air moves in, daytime highs may struggle to
climb above the mid 30s on Tuesday in the cold advection pattern.
With the heart of the cold airmass in place for much of the night
on Tuesday night, widespread teens are expected for lows.
The cold airmass will still be in place for Wednesday with many
locations likely struggling to climb above freezing. Temperatures
will likely moderate ahead of the the last clipper system of the
long term period on Thursday to around 5 degrees below normal,
with similar highs are expected on Friday.
&&
.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
ISSUED AT 622 PM EST FRI DEC 8 2017
We continue to see mid and upper level clouds from strong upper
level jet and disturbances aloft. The good news is we remain VFR
and this will be the story for the TAF sites through much of the
period. There is a chance of seeing moisture sneak into our far
southeast tonight which could lead to light snow. While this
could lead to localized issues, the models are not handling this
very well based on current observations leading to less
confidence. Another upper level disturbance drops southeast
Saturday afternoon, and we will see snow showers develop and move
across the area. This could lead to localized drops in CIGs and
VIS, but kept the TAF sites VCSH for now. Given some influence by
nosing high pressure think the winds will remain calm tonight, and
then winds will increase through the day Saturday out of the west
at 10 knots with gusts of 15 to 20 knots.
&&
.JKL WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
UPDATE...DJ
SHORT TERM...KAS
LONG TERM...JP
AVIATION...DJ
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
902 PM CST Fri Dec 8 2017
.UPDATE...
902 PM CST
No big changes planned to going forecast. Did low temps upward
just a bit tonight.
Potent jet is diving south into the Upper MS Valley region this
evening with RAP analysis suggesting 150kt+ 300mb winds at the
core with 125kt sampled on MSP`s evening sounding. Strong ascent
in the left exit region of this jet is resulting in strengthening
ascent and blossoming area of virga over northern IL. Radar
presentation would suggest that snow is very close to reaching the
ground if it isnt already over the northwest suburbs. Recent ACARS
soundings suggest the dry layer is only on the order of about
3-4kft deep and virga will likely be able to penetrate through and
saturate the low levels this evening.
Once low levels are saturated, expect light snow to develop withe
some bursts of occasionally moderate snow showers. Most intense
snow showers should occur near and just ahead of the approaching
cold front later tonight. See no reason to make any adjustments to
forecast snow amounts tonight with most areas likely to see a
dusting to an inch; though areas of NE IL and NW IN could see an
inch to perhaps 2" on the high end.
One change to forecast for Saturday morning was to add post
frontal snow showers and flurries in the CAA regime, particularly
eastern CWA in closer proximity to the cold upper low. Moisture
will be limited, but what little moisture there is should be
squeezed out fairly easily given the steep low level lapse rates
and strong CAA. Not expecting much more than isolated amounts of a
few tenths to maybe a half inch, but that should be spotty given
the expected isold-wdly sctd nature of the activity.
The same steep low level lapse rates and high RH in the low level
conducive for CAA driven snow showers over land will make for a
stellar lake effect snow (LES) set-up tomorrow. LES parameters
are quite impressive and it looks like an intense single band
could develop. Tropopause drops to near 500mb which will also
serve as the equilibrium level for lake induced convection (very
high for LES) and combined with 850-lake delta Ts approaching 20C,
suggests a very potent LES band should develop. Given the progged
parameters, if single LES band develops as expected, would expect
snowfall rates of at least 2" per hour (probably higher) in the
heart of the snow band. The unseasonably warm lake and deep
instability also raise the specter of potential isolated
thunderstorms with the LES band tomorrow.
Big question for snowfall amounts will be residence time of the
snow band at any given location. Hires models strongly suggest
that lake induced mesoscale enhancement of the surface low will
take place Saturday. The primary LES band is expected to develop
to the west of this low, with the low likely to result in some
meandering/wavering of the position of the band. Gave very strong
consideration to upgrading Porter to a winter storm warning, as
such an intense band could drop 6" of snow in just a couple hours.
If rates of that magnitude occur, impacts could be very
significant, particularly in light of the expected 30-45 mph winds
near the lake which could result in white-out/blizzard conditions
at times in the more intense snow squall(s). The primary factor
keeping me from upgrading to a warning at this time is the
uncertainty in location of the LES band. The operational HRRR
keeps it primarily east of Porter County, but many other models
have it farther west and keep Porter County well within the line
of fire.
- Izzi
&&
.SHORT TERM...
112 PM CST
Through Saturday...
Expect a burst of dry snow tonight area wide, mainly after
midnight. Gusty northwest winds will arrive Saturday morning
creating low lying drifting snow. Northwest Indiana, particularly
Porter County, will likely see some lake effect snow, along with
blowing snow on Saturday.
We did not see any big changes in the 12z guidance to change the
message of a quick hitting band of accumulating snow overnight.
This system is still across southern Canada, but will make a bee-
line southeast into our area tonight, then quickly exit east on
Saturday. This will be a drier snow with fairly decent -12 to -18C
omega shifting through the area just ahead of a cold front
overnight. Areas east stand chance of a bit more accumulation.
Expect a several hour period of snow, and with cold pavement,
accumulation will happen quickly once the initial top down
saturation occurs this evening. The main snow will occur after
midnight. This looks like a solid 1-2" event across the center of
the CWA, with lower totals west (north central Illinois and
across Central Illinois) and higher numbers possible east across
Indiana. A few showers will linger Saturday morning, though
accumulations would be limited to a dusting or a few tenths.
Concerns on Saturday will shift to gusty northwest winds,
drifting/blowing snow. In northwest Indiana, lake effect snow is
expected. Earlier in the shift, we added a winter weather
advisory for Porter County beginning tonight and ending late
Saturday. The synoptic snow does not appear to be significantly
different than other areas, although northwest Indiana is in a bit
more favored track for initial synoptic snows. After these few
inches of snow fall, strong winds (gusts to 30 mph or so) will
occur behind the snow as a compact surface low will pass right
over southern Lake Michigan. Forecast soundings across northwest
Indiana depict several hundred J/Kg of lake induced CAPE, with
this CAPE square through the dendritic growth region, and
reasonably high equilibrium levels. The heaviest lake effect snow
(which is typically highly variable dependent on location) will
fall into LaPorte and counties east where stronger convergence is
depicted, but there is a decent signal into Porter county for at
least a time. These combined concerns of strong winds, especially
for northern portions of the county seem to warrant an advisory.
KMD
&&
.LONG TERM...
147 PM CST
Saturday night through Friday...
A fairly active weather pattern is still expected for the next
several days, with periodic chances for pcpn and generally below
normal temperatures.
By Saturday evening, any remaining snowfall will likely be some
lingering lake effect snow for northwestern Porter County as low
level winds back from nwly early in the evening to swly by morning.
Wind speeds should also be on a diminishing trend as a weak ridge of
high pressure builds across the midwest. The upper level pattern
will continue to trend toward high amplitude and slow progression
with upper ridging building over the wrn 1/3 of the CONUS and broad
troughing over the ern 2/3 CONUS. The slowly progressive, high
amplitude long-wave pattern lends relatively higher confidence in
the overall trends of the long term forecast period, with persistent
nwly flow aloft keeping the region under persistent, deep layer
cold advection. This would suggest that the trend of below normal
temperatures will continue through the period. Toward the end of
the forecast period, from Wednesday through Friday, the long term
models are trending a bit warmer than the large scale pattern would
suggest and feel that model guidance temperatures are a bit too
high, so have trended the temperature forecast a few degrees lower
than much of the model guidance would suggest. The other, more
complicated, problem for the extended forecast period will be pcpn
chances. Under nwly flow aloft, pcpn chances will largely be driven
by shortwaves dropping out of Canada and tracking across the
midwest. After a dry period Saturday night and sunday, general
forecast confidence is relatively high in periods of pcpn, likely
snow, from Monday through the remainder of next week. There is
relatively good agreement among the models in the idea of a quick
moving clipper bringing another round of snow to the region. The
fast nature of the system would suggest that any new snow
accumulations should be light. After Monday, confidence in the
exact timing, track and intensity of individual shortwaves
diminishes as model solutions of the finer scale details diverge.
So, will continue the idea of periodic chances for pcpn, which
should mostly be snow. Each shortwave dropping out of Canada will
bring a reinforcing shot of cold air, helping to maintain the
general trend of below normal temperatures through next week.
&&
.AVIATION...
For the 00Z TAFs...
The main concern is the period of snow tonight. Light accumulations
of up to an inch or so of dry/fluffy snow are expected for the
eastern terminals, with less west. There is some uncertainty on
the snow start time later this evening, which will depend on the
low levels fully saturate. However, confidence is medium-high in
timing of the highest rates, which will be up to about 3 hours
after 06z. Looking at upstream observations, visibility of 1 mile
or less is probable during the period of highest rates, along with
brief low MVFR to IFR CIGs. The steadier snow will quickly taper
off prior to 12z, with on and off snow shower activity likely for
the eastern terminals through mid day/early afternoon. There looks
to be a window where 2-4 hours of more robust snow showers are
possible, starting in the mid to late AM. In addition, lake effect
snow could set up near GYY, but most likely just east.
Southwest winds will continue in advance of the clipper low
pressure system this evening and much of the overnight. Winds will
then shift to strong northwest Saturday morning as the low tracks
to western lower Michigan, with gusts in the 25-30 kt range,
possibly higher at times especially at GYY. Any lingering MVFR
CIGs will clear by Saturday evening, along with diminishing west-
northwest winds.
Castro
&&
.MARINE...
200 PM CST
Headlines...A Gale Warning is now in effect for the southern 2/3
of Lake Michigan, from Saturday morning into Saturday evening.
North to northwest winds are expected to increase rapidly late
tonight as deepening low pressure lifts through the eastern Great
Lakes and high pressure builds across the Upper Missouri Valley.
The far northern portions of the lake will likely remain below
Gale Force as the gradient should be a little weaker there. The
overall period of gale potential should be relatively short as the
ridge builds across the western Great Lakes by Saturday evening,
allowing winds to diminish and back to southwesterly.
The series of low pressure systems will continues through next
week. Another weak low will form near Lake Superior Sunday
morning followed by a stronger low that moves over Western Great
Lakes Monday afternoon. Guidance differs if the low will pass over
central Lake Michigan or south of the lake entirely, which would
have some impact on the forecast wind trends. However, at this
time, it looks like Gales are again possible behind the low
Monday night into Tuesday. Another high pressure ridge will
quickly pass over the western Great Lakes Wednesday bringing a
short period of diminishing winds, before ramping up again as the
next low approaches the Great Lakes late next week.
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Winter Weather Advisory...INZ002...midnight Saturday to 6 PM
Saturday.
LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...6 AM Saturday to 9
PM Saturday.
Gale Warning...LMZ743-LMZ744-LMZ745...9 AM Saturday to 9 PM
Saturday.
Small Craft Advisory...IN nearshore waters until 9 AM Saturday.
Gale Warning...LMZ777-LMZ779...6 AM Saturday to 9 PM Saturday.
&&
$$
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Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
420 PM MST Fri Dec 8 2017
.DISCUSSION...(This Evening Through Friday)
Issued at 219 PM MST Fri Dec 8 2017
Strong upper ridging persists across the western CONUS with deep
upper trough across the eastern half of the country. Surface low
sits over northern MN with a strong pressure gradient continuing
across western SD. Winds across the area are generally northwest 20
to 30 mph with gusts to 40 mph. The exception is across the plains
just east of the Black Hills where winds obs show 25 to 40 mph with
gusts up to 55 mph. Winds will decrease over the next two hours as
the overnight inversion begins to set up. Upper ridging over the
next several days will move very little...resulting in not much
change in the weather pattern through much of next week. The
baroclinic zone will stay east of the area with temperatures
generally in the 40s to near 50. However warmer temps are still
progged to push into the region through Saturday and Sunday...with
highs in the 50s (possibly even low 60s Sunday). Winds will also be
persistent through the next several days. Breezy northwest winds
will continue through the weekend. A clipper system will cross the
area Monday driving in colder temps in the 30s and 40s...along with
strong northwest winds. It looks like winds Monday could reach
advisory criteria...and possibly high wind warning. Will have to
keep an eye on models trends for that system. Rest of the week will
be quiet with highs in the 40s and not-as-breezy winds. Next chance
for snow showers will be toward the end of the work week.
&&
.AVIATION...(For the 00Z TAFS Through 00Z Saturday Evening)
Issued At 417 PM MST Fri Dec 8 2017
Gusty west/northwest winds will continue through the period, esp
around the eastern Black Hills. Low-level wind shear is expected
at RAP later this evening into early Saturday morning with
northwest winds at 50 knots around 2000 feet as surface winds
lessen to under 20 knots. Otherwise, VFR conditions will prevail.
&&
.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Wind Advisory until 5 PM MST this afternoon for SDZ026-031-072-
073.
WY...None.
&&
$$
DISCUSSION...13
AVIATION...JC