Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/10/20
Area Forecast Discussion
National Weather Service Burlington VT
951 PM EST Wed Dec 9 2020
Snow is expected to continue this evening, and become confined
to the higher terrain on late tonight into tomorrow. Dry weather
returns on Friday but precipitation chances will be increasing
during the latter part of the weekend. The leading edge of
precipitation could produce a brief wintry mix late Saturday
evening, which will transition to all rain heading into Sunday.
The weather system on Monday with lingering, terrain driven
shower activity becoming snow. Then a brief dry period is
expected Tuesday into much of next Wednesday.
.NEAR TERM /THROUGH THURSDAY NIGHT/...
As of 948 PM EST Wednesday...Forecast continues to remain in
good shape. Area webcams and radar show scattered light snow
showers across the area, mainly confined to the western slopes
and higher terrain, northeastern Vermont and the northern
Champlain Valley. Things should continue to taper off over the
next several hours with between a dusting and half inch expected
by sunrise. No real changes to the forecast were needed.
Previous Discussion...Not much has changed as light snow
composed of fine crystals continues to fall. The forecast
remains generally on track, though snow has continued to
struggle to make its way into eastern Vermont. We should see
increasing lift as a result of cyclonic vorticity advection over
the next few hours, and forecast soundings suggest we become
supersaturated with respect to ice with deepening snow growth
zones. This should result in some better dendrites. However,
surface temperatures have warmed to right around freezing, which
could make accumulating a bit more challenging. Driving is
likely a bit slick where roads are untreated with pavement
temperatures right around 30 F over Vermont. RAP mesoanalysis
depicts a relatively thin axis of better moisture in the DGZ and
there`s not much in the way of forcing for ascent, which is
supportive of continued light snow. Pocket of 700mb moisture
begins to thin, tonight. Snow showers becoming lighter and
confined to the high terrain.
Temperatures are not likely to fall much this evening with no
real change in airmass and abundant clouds. Thus, lows around 30
are expected. Any lingering snow showers in the valleys should
end early Thursday morning, and then we should see snow finally
exit the mountains by noon, with the last vestiges in the
Northeast Kingdom thinning near 3 PM. Surface high pressure will
sit to our south, but it has a ridge axis that extends
northwards to our west that will keep our winds out of the
northwest most of the day. This should keep temperatures in the
30s. Pockets of high clouds will skirt the region, and winds
should gradually turn out of the south overnight on Thursday.
This will keep lows from falling too far. Expect mid 20s in the
valleys, and upper teens to around 20 across cold hollows.
.SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/...
As of 317 PM EST Wednesday...Forecast will trend warmer for
Friday as ridging off the Mid-Atlantic Coast shifts eastward and
return southwesterly flow develops over the North Country.
Highs will be in the upper 30s to mid 40s throughout the area.
The day will start off with a few breaks in the low clouds and
some peaks of sun, but by the afternoon hours will see more low
clouds work into the area as several weak frontal boundaries
move through. May see a few light rain/snow showers during the
afternoon and evening hours Friday, but the majority of the
period will be dry. Low clouds with the potential for a
scattered light shower or two will persist into the overnight
hours. Lows will be in the upper 20s to low 30s.
.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
As of 317 PM EST Wednesday...Continuing to watch a low pressure
system in the long term forecast that will impact the forecast
area this weekend. The low will initially form over the southern
Plains Friday, lifting northeastward through the MidWest and
then weakening as it travels into Ontario/Quebec by Saturday
into Sunday. This track would keep our forecast area mainly in
the warm sector of the cyclone. Initial warm front will push
through Saturday afternoon into Saturday evening, with some
potential for a quick shot of some messy mixed precipitation.
Still a little early to get a proper handle on surface
temperatures and the evolution of the warm nose associated with
the system, which will directly impact icing potential. However,
can`t rule out a favorable thermal profile for some freezing
rain, especially Saturday evening. Have kept mention of any
freezing rain out of the forecast at this point with too much
uncertainty around temperatures in the lower-levels, but will
continue to watch trends and may need to add it in in subsequent
Temperatures will warm through Saturday night to near to above
freezing throughout the forecast area, so will see a period of
rain for the bulk of the event Saturday night through Sunday as
we are solidly in the warm sector of the cyclone. The system`s
cold front will move through late Sunday/early Monday night,
allowing for a transition to some waning snow showers on the
back edge of the system.
Overall, main threat of the system that we will continue to
watch will be the icing potential for Saturday evening. No hydro
concerns at this point and, given the weakening nature of the
system as it tracks into Canada, not seeing much of a wind
The weather will turn drier as we head into next work week with
a cooling trend going into midweek.
.AVIATION /03Z THURSDAY THROUGH MONDAY/...
Through 00Z Friday...Light snow continues in the Champlain
valley, and is slowly coming to an end across the rest of our
area. Most locations still have MVFR ceilings, despite snow
showers coming to an end. Exceptions are SLK which is IFR and
PBG which is VFR. Will take awhile for ceilings to improve,
likely staying MVFR through the overnight. Snow showers will
linger the longest at BTV and SLK. Winds are currently southerly
and will be veering around to westerly overnight. Remaining
snow showers should dissipate around 12-15Z, and we will see
ceilings begin to improve as well towards 2500-5000ft agl.
Thursday Night: VFR. NO SIG WX.
Friday: Mainly VFR, with local MVFR possible. NO SIG WX.
Friday Night: Mainly MVFR, with local IFR possible. NO SIG WX.
Saturday: Mainly VFR, with local IFR possible. Chance RA, Slight
Saturday Night: Mainly MVFR and IFR, with areas VFR possible.
Likely RA, Likely SN.
Sunday: Mainly MVFR, with areas IFR possible. Likely SHRA.
Sunday Night: Mainly MVFR, with local VFR possible. Chance SHRA,
Monday: Mainly MVFR, with local VFR possible. Chance SHSN, Slight
Area Forecast Discussion
National Weather Service Louisville KY
907 PM EST Wed Dec 9 2020
Issued at 905 PM EST Wed Dec 9 2020
Model time-height sections from the GFS and to a lesser extent the
RAP show a deck of mid to high-level clouds coming into the region
late tonight. Looking at upstream satellite imagery, starting to see
some formation over Missouri and Arkansas. As the night goes on,
model RH fields show increasing saturation in the 200-400 mb layer,
particularly the GFS1hr. CMCreg/NAM/RAP are not as wet, so will just
go with a scattered high deck for now. The other concern to watch
for is fog formation. Statistical guidance is not too gung ho on the
potential, but they do show some conditional category 1`s for
ceilings, indicative of a very low cloud layer that sometimes can be
fog. Will focus on areas west of I-65 for the next couple of hours
for patchy fog potential, but then as some high clouds build in,
think it will shift to patches more east of I-65.
.Short Term...(This evening through Thursday)
Issued at 251 PM EST Wed Dec 9 2020
Things remain quiet in the short term with dry and notably mild
conditions continuing. Current NW flow aloft will transition to a
brief zonal flow tomorrow, with only some upper clouds around at
times. Skies should be mostly clear overnight, and with light and
variable winds the good radiational cooling could allow for some
patchy fog development. Have included this fog mention mainly east
of I-65. Otherwise, expect lows in the low to mid 30s in most spots.
Highs on Thursday are expected to be even milder than today, with
the mercury topping out in the 60 to 65 degree range in most spots.
Our NE CWA could hang onto the upper 50s, but still a couple
degrees milder than we are seeing today.
.Long Term...(Thursday night through Wednesday)
Issued at 235 PM EST Wed Dec 9 2020
Friday continues to look like the warmest day of the 7-day forecast
period as the region sits well within the warm sector of a warm
front that will be near the Great Lakes region. While clouds will
gradually increase through the day Friday, modest southwesterly flow
should result in high temperatures climbing into the low and mid 60s.
Showers and even a few rumbles of thunder will move in for Saturday
and persist for a good portion of the day. Despite the widespread
rain showers and clouds, temperatures will remain fairly mild until
the front passes through, which may not occur until closer to
Long range guidance has started to come into better agreement for
the Sunday through early next week timeframe. Appears now that any
systems that develop in the Gulf will be suppressed far enough south
to limit any meaningful precipitation chances and accumulations in
central Kentucky. Will still hold on to some low-end PoP chances for
portions of central Kentucky during this time since guidance could
shift things again in the near future, but current trends suggest
drier/cooler weather to prevail after Saturday`s fropa.
.Aviation...(00Z TAF Issuance)
Issued at 630 PM EST Wed Dec 9 2020
Nice night across the region with winds starting to become more
light and variable. They should become southeasterly later tonight
and then southerly during the day Thursday. We`ll see a deck of
higher clouds move into the region overnight. Even with that deck
there still are some question marks with regard to fog potential
early Thursday. Have gone ahead with HNB/LEX tempo groups for MVFR
visibilities, given a weak signal in LAMP probability guidance and
forecast low temperatures a little cooler than this afternoon`s
Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
827 PM CST Wed Dec 9 2020
827 PM CST
The forecast updates for this evening have only been a nudging
down of the low temperatures in some spots and tweaks to sky
cover, the latter of which has a touch more emphasis than usual
with a chance for seeing the aurora borealis (Northern Lights).
The inclusion of patchy fog into the forecast remains warranted
too, so no changes there.
A clear early evening with surface high pressure sprawling in
provided rapid decoupling. Both Aurora and Rochelle airports
already dropped to 30 degrees as of 3 p.m., quite a fall from 3
p.m. readings in the lower 50s. A patch of some translucent to
somewhat opaque cirrus near 20,000 ft is over the northeastern
quarter of Iowa and drifting toward the area, but satellite
imagery has indicated a gradually thinning trend. Some cirrus
behind this toward Minneapolis was sampled well by the MPX 00Z
sounding and indicated the moist layer being quite thin. So while
some cirrus is likely to drift over the area along/north of I-80
between 10 p.m.-3 a.m. and possibly temporarily obscure a sky view,
it looks like the cirrus may be often somewhat thin and not to
persist that entire time over any one place. Temperatures may
wobble some under those clouds, but did lower lows a bit given
trends, and expect there to be several outlying sites to reach the
VWP data indicates in the past couple hours the winds having
really fell off above the inversion, and the RAP forecast
hydrolapse indicates a favorable profile for shallow fog. Despite
the sharp temperature fall, dew points have generally been
falling too though. With the cirrus drifting overhead for part of
the area, that looks to probably limit any widespread fog threat,
at least that to majorly public-impacting. Some patches of fog to
dense level are certainly possible in this regime, especially if
the cirrus presently over southern Minnesota does not make it here
intact thus yielding more clearing in that 2-6 a.m. time.
340 PM CST
Through Thursday night...
Quiet weather will prevail through Thursday as upper ridging builds
overhead. The main concern in the short term is the potential for
shallow ground fog tonight, especially outside of the metro.
Clear skies have allowed temperatures to warm steadily into the
low to mid 50s across the area today. Have nudged up overnight low
temperatures accordingly. There still remains a fairly decent
signal, especially in the hi-res guidance for shallow ground fog
development overnight into tomorrow morning. Surface high
pressure will be centered over the area with light winds and clear
skies. Have introduced a patchy fog mention with this forecast
package, although this could be a bit conservative. Will continue
to monitor the potential for greater coverage.
Given the warmth from today, expect another warm day on Thursday
with high temperatures in the low to possibly mid 50s. The upper ridge
begins to shift to the east Thursday night with a dry cold front
also moving through the area. Expect clouds to increase behind
the front as our next system approaches from the southwest.
313 PM CST
Friday through Wednesday...
The primary forecast challenges in the extended period continue
to focus on the evolution of a storm system that is likely to
impact the area late Friday through Saturday night. This storm
system promises to bring a significant amount of precipitation to
the area, but mainly with most of it falling as rain over central
and eastern IL into northwestern IN. However, the potential
exists for significant snow accumulations for at least parts of
far northwestern into far north central IL and points northward
into WI. Therefore, we could see a very sharp gradient between
significant snow, and little to no snow somewhere right across
northern IL, possible even nearing parts of the western/
northwestern suburbs of Chicago.
Model and ensemble guidance is in decent agreement with surface
low pressure taking shape over the southern Plains on Friday in
response to the approach of the upper level low currently off the
Baja of California coast. The exact track of the surface low is
always key to determine where the accumulating snow will fall.
Current indications continue to support a northeast track of the
low from near STL early Saturday morning, to northwestern IN early
Saturday afternoon. This track, would favor the main area of
accumulating snowfall to be roughly across my northwestern areas,
namely the Rockford metro area and possibly as far southeast as
parts of the far northwestern Chicago suburbs. Mainly a rain event
looks to be in store for much of the rest of the region. We will
have to continue to watch this closely as even a minor shift in
the expected track could change this scenario, so stay tuned!
While the placement of heaviest snow accumulations continues to
be unclear, there is high confidence in a good amount a
precipitation over the area late Friday through Saturday evening.
As noted in previous discussions, an impressive northward surge of
Gulf Moisture is expected with the approach of the area of low
pressure. This is noted by precipitable water values progged to
climb to near 1" over southern sections of the area, which is
above the 90 percentile for this time of year. This in combination
with what looks to be good upper level jet support, with the
possibility for upper level jet coupling all point towards an
impressive precipitation event for the area. For these reasons,
widespread amounts of 1 to perhaps close to 2 inches of liquid
equivalence is probable across the area through Saturday night.
The precipitation looks to onset across the area Friday late
morning and into the afternoon following a surface cold frontal
passage. A lower level baroclinic zone will then sag over the
area setting the stage for rain to develop during the day. The
rain then looks to mix with and change to snow by early Friday
evening somewhere over far northern IL and then continue at times
through Saturday as the column cools. The period of heaviest
precipitation looks to be from late Friday night through the mid
afternoon hours on Saturday as the best dynamics line up
overhead. The precipitation then looks to gradually come to an
end from west to east Saturday night into early Sunday morning,
likely ending as a little snow for most areas, though with little
Some potential will exist for some weak lake effect into parts of
northwest IN through midday Sunday, though forecast soundings
indicate rather unimpressive parameters with no real arctic air
intrusion and it currently looks as if winds should turn
northwesterly enough by afternoon behind the departing system to
shift any threat east of our forecast area. Though models continue
to show some significant details with the pattern early next
week, a couple days of seasonably colder weather appear in store
for the region Monday and Tuesday as weak surface high pressure
spreads into the area.
One other note with the weekend storm, is that as winds turn
northerly and increase late Friday night through Saturday, there
may be a period of lakeshore flooding potential along the IL and
IN shores of Lake Michigan. The greatest threat, with waves 8+
feet, would likely be Saturday afternoon and evening.
For the 00Z TAFs...
514 PM CST
* Only item of note is possible MVFR BR at outlying terminals
Mainly quiet aviation conditions are in store for the next 24-30
hours. Patchy FG/BR appears probable south and west of the terminals
late tonight into Thursday morning. Best chance for IFR VSBY is
south of the outlying TAF sites. Maintained TEMPO MVFR VSY at DPA,
RFD, and GYY. Chicago sites should stay VFR. Light/VRB winds will
become south and pick up some mid-late Thursday morning. Some mid
to high teens kt gusts are possible in early to mid afternoon.
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Area Forecast Discussion
National Weather Service Marquette MI
645 PM EST Wed Dec 9 2020
.SHORT TERM...(This evening through Thursday)
Issued at 403 PM EST WED DEC 9 2020
WV imagery and RAP analysis indicated a split flow pattern with
mid/upper level ridging through the Plains and wnw flow prevailing
through the northern Great Lakes. Upstream shortwave troughs were
moving through southern Saskatchewan and into northwest Montana. At
the surface, northwest winds persisted between a trough over Quebec
and high pressure building eastward from the Plains. Vis loop showed
clear skies over most of Upper Michigan with clouds slowly receding
over the east. Additional mid/high clouds supported by the developing
WAA pattern were quickly spreading ese through northern MN.
Tonight, with the ridge building into the area and a period of clear
to partly cloudy skies, temps should drop off into the low to mid
20s inland with readings closer to 30 near the Great Lakes. If the
mid clouds thicken more quickly overnight, temps may slowly rise
late. Potential northern lights viewing may also be complicated by
the clouds moving in near or after midnight.
Thursday, as the northern stream shrtwvs and western Dakotas sfc
trough approach with moderate to strong 290k-300k isentropic ascent,
pcpn should also expand from far ne MN into northern Lake Superior.
However, expect the pcpn to remain north of the the all but the tip
of the Keweenaw where slightly chance POPs for rain/snow was
mentioned. Winds veering to the northwest late in the day as the
trough moves in could also bring some light lake enhanced pcpn to
the Keweenaw. Otherwise, expect continued mild conditions with
mild southerly flow ahead of the trough with highs in the low 40s.
.LONG TERM...(Thursday night through Wednesday)
Issued at 343 PM EST WED DEC 9 2020
The medium to extended forecast period will usher in a split flow
regime where the polar jet assists with surface low pressure
development north of the area, and the subtropical jet will assist
with mid-latitude surface low pressure development to the south of
the area through the weekend. This isn`t to say we will have no
weather over the area at all, but a phased signal would need to be
achieved with deep troughing over the central and northern CONUS for
impactful weather to occur in our CWA. Ensembles from the GEFS and
GEPS do begin to signal more troughing over the CONUS from this
weekend through portion of next week, but is not expected to assist
with cyclogenesis directly over the CWA during this period.
Looking more in depth for the forecast brings to light a few details
to note. A shortwave trough Thursday night through Friday morning
will bring in colder surface temps behind a cold front. Model
soundings show low level saturation through approximately 3-5kft
before the inversion takes hold and dry air infiltrates the area.
Precipitation chances look low at this time to include in the local
forecast. The cold front will be on the order of approximately 7-10
degrees colder for Friday afternoon highs compared to the previous
The next weather disturbance of note to mention will be the
evolution of the surface low as lee-side cyclogenesis begins to
occur over the rolling plains of TX/OK Friday. This surface low is
expected to deepen by approximately 10mb over the course of Friday
morning through Saturday afternoon as it treks to the northeast over
the Great Lakes region. Model consistency has been congealing into a
better signal the last 24-36 hours, but the latest iteration of the
12Z NAM has introduced an interesting possibility albeit as the
outlier to other guidance. The 12Z NAM has the surface low center
just east of Cedar Rapids, IA instead of the southern tier of Lake
Michigan by 0Z Sunday. This is about 200 miles further to the west
of the other guidance solutions for a surface low center, and could
present the opportunity for more accumulating snowfall on our
southern portions of the CWA if the surface low continued to
propagate northeast from there. However, did not change forecast
evolution with regard to snowfall chances yet due to it not lining
up with other guidance, in addition to the 84 hour limit for the
NAM. Wrap around winds will likely create some lake effect snow
showers for the CWA, but snowfall totals will be limited based off
the present guidance available.
For Monday through the middle portion of next week, an additional
shortwave disturbance is anticipated to move over the Great Lakes
area. 850mb temps looks cold enough to support an all snow event for
the lake effect snowbelts, but will be conservative on the forecast
as this is 120+ hours out as of this forecast package issuance.
.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 644 PM EST WED DEC 9 2020
VFR conditions are present at all terminals, but mid level clouds
are moving into the area and should fill in overnight. The feature
of note during this TAF period is a cold front expected to move
across the area late Thursday afternoon and into Thursday evening.
The cold front should abruptly end the prolonged period of VFR cigs.
.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 403 PM EST WED DEC 9 2020
NW winds will subside below 20 kts this evening as a high pressure
ridge passes by to the south. Thursday night a cold front will sweep
across the lake bringing more 20-30 kt NNW winds into Friday
morning. After that, Saturday remains the next period to watch.
Models seem to have settled on a farther south track with the low
pressure system that will be moving through the Great Lakes, which
means NE winds Saturday and Saturday night should remain in the 20-
30 kt range with gales looking unlikely. Winds are expected to back
around to NW and relax below 20 kts for Sunday.