Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 02/08/20
Area Forecast Discussion
National Weather Service Denver/Boulder CO
303 PM MST Fri Feb 7 2020
.SHORT TERM...(This evening through Saturday)
Issued at 300 PM MST Fri Feb 7 2020
A jet streak in the NNW flow aloft continues to sustain heavy snow
and strong winds in the mountains, as well as bands of snow along
the urban corridor and out to the adjacent plains. In particular,
a long sustained narrow snow band from Grand County stretching
southeast across Jefferson and Douglas counties...which continues
all the way down into Texas, has produced over a foot of snow in
our forecast area today. Models haven`t performed so well with
this event, typical of jet induced banded snow, so just need to
look at the big picture. With the speed max currently laying
across the Front Range Mountains and slowly drifting southeast,
should see bands continuing late this afternoon into the early
evening. This can be seen with cloud tops cooling and snow bands
already reforming over the mountains and spreading southeast over
the urban corridor. While confidence remains low in the models,
several show the long sustained band moving slightly north and
east, with additional bands pushing across the metro areas through
early this evening. Therefore, will continue the advisories out
for the plains and foothills, with snow expected to decrease and
diminish in the late evening. The southern foothills Advisory will
likely be able to be cancelled by later this evening. RAP is
showing a second speed max pushing in that may sustain some light
snow over the plains through midnight, but will have to keep watch
on the model trends.
The heavy snow, strong winds and blowing snow will continue to
bring terrible travel conditions to the high country through the
rest of the evening with another 3 to 6 inches by midnight. Rates
and winds will be decreasing after this, but will likely still
see another 1 to 4 inches.
Flow will turn more westerly for Saturday with a lee surface
trough forming. Brief upper ridging across the area will bring
clearing skies, and warm advection combined with downsloping winds
into the surface lee trough will bring much warmer temperatures
compared to today. Light snow may still occur over the far
northern mountains in the morning, before snow will increase
in the afternoon ahead of the next system dropping down along the
Northern Rockies. The jet ahead of this system and the digging
surface trough will increase winds across the mountains again and
down into the foothills and possibly the western suburbs. Blowing
snow will likely become an issue again even with little snow, if
any, falling yet.
.LONG TERM...(Saturday night through Friday)
Issued at 300 PM MST Fri Feb 7 2020
Next storm system is quick on the heels of this one, already
dropping southeast off the coast of British Columbia. This system
will continue to move southeast into the Northern Rockies on
Saturday. QG fields show most of this energy splitting as it moves
into the Central Rockies by Saturday night. However, we`ll still
have the support of the right rear entrance region of an upper
level speed max stretching across Colorado. This lift, along with
increasing mid level moisture and modest orographic lift, will be
enough to get another round of snow going in the high country
Saturday night. We expect periods of mainly light snow to then
continue through Sunday and Sunday night under the nearly
persistent right entrance region of the upper jet. Despite the
weakening orographic component during this time frame, given the
jet`s proximity, can`t rule out a few heavier embedded bands.
On the plains, we`ll see a cold front move across late Saturday
evening, with shallow upslope developing along the Front Range.
That upslope gradually deepens through Sunday, so we`ll also see
snow develop across the foothills and nearby adjacent plains and
I-25 Corridor. Right now, initial accumulation projections
Saturday night through Sunday night would be 4-10 inches in the
mountains, and 1-5 inches along the Front Range.
By Monday, we`re looking at another piece of energy dropping
southward across the Northern Rockies and Northern High Plains.
This will probably push a weak reinforcing cold front across
northeast Colorado, and keep at least a chance of light snow in
the forecast. Temperatures will stay well below normal, with highs
struggling to reach freezing.
Tuesday should feature dry conditions with a gradual warming
trend, but then we expect another weak trough late Wednesday or
Thursday with cooler temperatures and a chance of light snow.
Friday should be in between systems, but another storm system will
likely impact the area by next weekend.
.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 300 PM MST Fri Feb 7 2020
Snow is expected to once again spread across the urban
corridor later this afternoon and into the evening. In general,
the snowfall rates be lighter compared to this morning, but MVFR
ceilings and visibilities will likely return to prevail through
this afternoon, with brief IFR associated with the bands of snow.
Additional accumulations will likely be in the 1-2 inch range
this afternoon into the evening, but it would be higher if a
heavier snow band develops right over the airport. Southerly winds
will become drainage this evening, increasing out of the west
Saturday mid morning as downsloping winds push east from the
foothills. Gusts up to 25 kts will occur near the foothills and
western suburbs. A cold front will likely push through Saturday
Winter Weather Advisory until midnight MST tonight for COZ030-
Winter Storm Warning until midnight MST tonight for COZ031-033-
Area Forecast Discussion
National Weather Service Des Moines IA
548 PM CST Fri Feb 7 2020
.DISCUSSION.../Tonight through Friday/
Issued at 309 PM CST Fri Feb 7 2020
.Overview...Lingering light snow or flurries, mainly over southeast
into central Iowa, will come to an end late this afternoon or early
this evening. Focus then is on the significant winter storm that
will largely be focused over northern Iowa and southern Minnesota
Saturday night into Sunday. Trends for the mid-week system are
beginning to favor perhaps a drier conditions, though there still
remains some uncertainty.
.Details...Early afternoon GOES-East clean IR imagery shows the
longwave trough still over the central and eastern US with the axis
now into the Ohio and Tennessee River Valleys. Shortwave trough
embedded in this longwave trough is beginning to move out of the
state to the southeast with light snow winding down over all but the
southeastern part of the forecast area. Forecast soundings from the
HRRR and RAP show low level saturation lingering with weak lift
within this saturation indicating some final flurries before all the
snow ends. This low level saturation seems to correspond fairly well
with the stratus as viewed in the Day Cloud Phase Distinction and
Day Snow Fog RGBs and also with surface observations that have -SN
reported underneath these clouds. Therefore, have added flurries on
the northwest side of the more persistent snow that will taper off
later this afternoon or evening.
Surface high pressure will build into the region tonight and move
out on Saturday as the flow in the mid-levels splits off into a
northern stream shortwave and the other piece forming a closed low
over the southwestern US. The northern stream shortwave with an
associated surface low pressure will advance towards the state
Saturday night with a majority of models (12z GFS/12z GEFS/12z
EC/12z CMC) having the low passing through northern if not central
Iowa. The one outlier is the 12z NAM that has the low passing along
the Iowa/Minnesota border. As the low approaches the state, low level
thermal lift will increase and there will be strong QG convergence
moving in overhead as well. Initially, cross sections do show dry
air in the low levels around 00z over northern Iowa, but that will
erode away between 3z and 6z. As saturation does occur, a strong
frontogenetical band, which will be maximized in the 850-700mb
layer, will move over northern Iowa and southern Minnesota. Cross
sections show strong lift in the dendritic growth zone over northern
Iowa starting around 6z (snow onset roughly) lingering to around
12z. If the NAM`s surface low track verifies, this strong lift may
be very focused over southern Minnesota. At this time range, there
appears to be high confidence in strong lift within the dendritic
growth zone for enhanced snow rates of 1"/hour over northern Iowa
and/or southern Minnesota. This is supported by this morning`s 12z
HREF probability of 1"/hr snow rates with isolated 2"/hr rates over
southern Minnesota. This area of enhanced snow rates and therefore
snow totals will be further refined as the event enters the forecast
window of the high resolution guidance (e.g. HRRR, RAP) in the
upcoming forecast cycles.
Writing about snow totals, the track of the low will be a key piece
and that is why it was mentioned earlier. While warmer air will be
drawn into the state in the low levels, this will allow for some of
the snow from Saturday night to changeover to rain during the day
Sunday. Further, a dry slot will move in over southern and central
Iowa on Sunday, though how far north will depend on the exact low
track. The dry slot will either 1) end ice introduction resulting in
a change from snow to freezing drizzle or rain to drizzle where it
is warm enough or 2) end precipitation all together. Either one of
these scenarios will serve to bring a sharp cut off to the higher
snow totals, which will likely be over northern Iowa. As the low
passes, winds will become from the northwest and turn gusty. How
gusty will again depend on low track with BUFKIT NAM profiles
suggesting wind gusts to around 35 mph while the BUFKIT GFS profiles
are not as strong. All the precipitation will come to an end by
Sunday evening as large scale subsidence and drying enter the area.
High pressure will pass through the region early next week followed
by a shortwave trough and associated moisture-starved, cold front
diving southward. The aforementioned closed low over the
southwestern US will begin to move eastward toward the central and
southern Plains early next week. There is still some disagreement on
how high pressure will move over the northern Plains, but there has
been a trend in the deterministic models of pushing the
precipitation farther south.
.AVIATION.../For the 00Z TAFS through 00Z Saturday evening/
Issued at 548 PM CST Fri Feb 7 2020
Lower cigs continue to hold on; especially northeast sites at
KMCW, KDSM, and KOTM where MVFR conditions continue for now.
Elsewhere, MVFR giving way to some VFR pockets with gradual
transition to VFR from KFOD northwestward. Have slowed departure
of lower cigs due to a band swinging southeast overnight. Will
monitor trends through 06z, but generally more improvement
expected between 06-12z most sites. /rev
Winter Storm Watch from late Saturday night through Sunday
afternoon for IAZ004>007-016-017.
AREA FORECAST DISCUSSION
National Weather Service Wilmington OH
932 PM EST Fri Feb 7 2020
Surface high pressure will bring a brief period of dry weather
later this afternoon into this evening. Another, quick moving
low pressure system will offer a chance for light snow late
tonight into Saturday. An active weather pattern looks to
continue this weekend into next week with several chances for
.NEAR TERM /THROUGH SATURDAY/...
Spotty snow showers developed this afternoon in the deep-layer
WNW flow, and a few of these remain across the area this
evening, though with very isolated coverage. Outside of these
snow showers, skies have remained generally clear, allowing
temperatures to drop off a little quicker than previously
anticipated. Min temps have been reduced by a few degrees,
particularly in the northern and eastern sections of the
A shortwave and associated surface trough are now moving
eastward through Illinois, soon to enter Indiana. As flow turns
briefly southwesterly ahead of the wave, a small 850mb theta-e
ridge will also be present as a result, with an area of snow
developing on the northeastern periphery of the theta-e ridge.
While the overall forecast thinking has not changed, the overall
forcing and thermodynamic environments are somewhat marginal,
and the method of forcing will not support a lengthy period of
accumulating snow -- likely only a few hours at any given point
on the map. While there will be saturation within the dendritic
growth zone, temperatures will cool through the layer, meaning
that the DGZ will not be particularly deep. Overall snowfall
forecast numbers on this forecast update are slightly lower than
with the previous forecast, but not enough to really change the
expected impacts, and thus the advisory will remain as placed.
Previous discussion >
Still a few patches of flurries this afternoon across the south
and east as influence from the low pressure slowly fades.
Beneath the dominant upper level trough, a weak 700-850 mb ridge
moves in from the west tonight, resulting in partly to mostly
clouds skies and initially dry conditions. Temperatures
currently range from 30-33 degrees this afternoon with perhaps a
degree of warming before temperatures begin to decrease. Expect
temperatures to drop into the mid to upper 20s with favorable
radiational cooling, especially across the northwest.
Late tonight, a respectable shortwave currently located over the
upper Mississippi Valley propagates eastward into the Ohio
Valley, promoting another round of light snow across the area.
Majority of the snow falls during the day on Saturday, but it
enters the area from the west late tonight. Winter weather
advisories have been issued for portions of SW Ohio, SE Indiana,
and N Kentucky starting late tonight.
Guidance suggest the best lift arrives in the tri-state area a
few hours before sunrise. Forecast soundings from the RAP show a
favorable thermodynamic profile with modest lift in the DGZ
between 6-8AM. This should result in robust dendrites (-10 to
-20C) that will accumulate quickly over a short period of time.
For this particular event however, the length of time limits
overall accumulations to 1-2 inches.
Regarding the winter weather advisory issuance...Similar to
Friday`s events, the timing of greatest snow rates and
deteriorating road conditions occur around sunrise leading to
hazardous travel. While the overall traffic flow should be
lower, those who do have to travel during the morning should
expect hazardous travel conditions. For this forecast package,
the advisory focused on the area that will experience the snow
during the optimum time frame (4am-9am) and also contains a
lower criteria (2" for advisory) due to the surrounding
terrain. Some areas outside the advised area (further north and
east)are forecast to see 1-2 inches but were not included in
the advisory due to higher tolerance and a less optimum time
frame (9am-1pm). Adjustments to the advisory are possible
overnight as the situation evolves.
.SHORT TERM /SATURDAY NIGHT/...
Saturday morning, snow is still expected to be ongoing across
the forecast area as the trough continues to move east. Through
mid-morning and early afternoon, lift gradually weakens and
snow rates decrease. Along with the decreasing snow rates, solar
radiation and warming temperatures help to limit overall
impacts for eastern/northern locations. With warming
temperatures, can`t rule out a period of rain-snow mix/rain to
across the south as precipitation moves east and out of the
area. Could still see light accumulations into the afternoon,
especially across the north, before the trough dissipates during
the evening hours as another weak ridge builds into the Ohio
Temperatures Saturday night drop into the mid 20s area wide.
.LONG TERM /SUNDAY THROUGH FRIDAY/...
Precipitation will develop on Sunday afternoon and evening ahead of
an approaching front. There could be some rain-snow mix or just snow
in northern counties at onset, but beyond then expect this to be a
rain event. The rain will push across the forecast area Sunday night
and move off to the southeast on Monday as a cold front moves
through the area.
High pressure will build in Monday night through Tuesday night.
There remains considerable uncertainty from Wednesday into Thursday
night. GFS continues to have a stronger northern stream which keeps
any southern stream system and its associated precipitation south of
the forecast area. However, there is enough spread within the GEFS
members to still suggest at least a chance of precipitation into the
area. ECMWF and it ensemble have trended further south with time
although it solutions are still further north than the GFS. There
are also timing uncertainty with when the southern stream energy
ejects out of the southwest. Thus will rely heavily on ensemble
means, using a blend of the GEFS and ECMWF ensemble. This suggests
that there is still a good chance of precipitation Wednesday night
into Thursday. Expect rain in southern counties with a rain-snow mix
or perhaps even all snow further north as uncertainties in solutions
also impact thermal profiles and thus precipitation type.
There is general agreement that high pressure will be building in by
Friday with slightly below normal temperatures.
.AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...
The main concern for this set of TAFs is the snow moving into
the area late overnight into tomorrow morning.
While generally VFR conditions are expected going into the
overnight hours, snow will overspread the TAF sites from
southwest to northeast. This snow will bring several hours of
MVFR visibilities and MVFR to IFR ceilings, and it is likely
that most of the TAF sites will also experience an hour or two
of IFR visibilities as a result of the snow. Once the snow comes
to an end, some persisting MVFR visibilities and low-end MVFR
ceilings may continue for a while, and a few snow showers could
still impact the Dayton and Columbus TAF sites through early to
mid afternoon. Ceilings will gradually improve late in the day,
with VFR conditions expected some time after 00Z.
OUTLOOK...MVFR conditions are possible again on Sunday night
into Monday with rain. MVFR ceilings may continue into Tuesday
OH...Winter Weather Advisory from 3 AM to 10 AM EST Saturday for
KY...Winter Weather Advisory from 3 AM to 10 AM EST Saturday for
IN...Winter Weather Advisory from 3 AM to 10 AM EST Saturday for
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
533 PM CST Fri Feb 7 2020
236 PM CST
A light snow dusting tonight, mainly along/south of I-80 with
some flurries north, is the item of interest in the short term.
Lake effect snow had dissipated across northwest Indiana as
surface ridging scoots into the area. Aloft in the mid levels, a
low amplitude positively tilted trough is angled across Iowa this
afternoon and will gradually move across the forecast area
tonight. This short wave trough is emphasized some by a vorticity
maximum (vort max) near the Iowa/Missouri border that will move
slightly south of east before turning more due east tonight. It is
near this vort max where differential vorticity advection and a
pocket of isentropic lift will bring periods of light snow. For
our CWA, that`s mainly along/south of a line from Streator IL to
Rensselaer IN. A small area of one half to on the high end one
inch may fall in that far southern part of the CWA helped by the
lift collocated with the dendritic growth zone (-12C to -18C).
North of there, forcing is much more transient and weaker along
with the fact the low level air is significantly dry (2 pm T/Td
spreads are 25 to 35 degrees F). This should result in mainly just
transient flurries and likely no accumulation. Any light snow
from this system should end early Saturday morning.
Upstream conditions in Iowa and Minnesota would point toward
broken stratocumulus clouds likely during Saturday, although they
may be thin with some holes at times in mainly northeast Illinois.
These clouds will reside in the vertical thermal profile near -10C
per RAP and NAM soundings, with some slight cold advection
induced low-level instability. That could bring some flurries.
Highs on Saturday with the clouds look to be around 5 degrees
cooler than today.
238 PM CST
Sunday through Friday...
The Madden Julian Oscillation (MJO) is expected to become active
in phase 6 (Western Pacific) through next week. Teleconnections
related to an active MJO in this phase tends to favor a negative
PNA-type pattern (Upper Trough in the west, ridding in the east)
across the CONUS. For this reason, the weather pattern across the
central CONUS is likely to remain active, but with temperatures
near seasonal averages in our area.
The first weather system of note is hybrid clipper type system
talked about in the previous discussion. This system is expected
to track eastward from the northern Plains to the western Great
lakes region late Saturday through early Sunday evening. A lower
level southerly wind response is likely to occur in advance of it
across much of the Mid-Mississippi Valley and lower Great Lakes
into early Sunday. This will in turn set up a large region of
isentropic upglide (warm air advection) over the region into
Sunday, thus setting the stage for a period of precipitation into
our area for Sunday.
The current track of the associated surface low suggests that the
band of heaviest snowfall of 6"+ will occur across parts of
southern MN and northern IA, eastward into adjacent areas of WI
Saturday night into early Sunday. However, farther southeast into
our area lighter total snow amounts are expected. It appears that
the snow could begin to onset across northern IL by early Saturday
morning, possibly just prior to daybreak in parts of north
central IL. This initial period of snow early Saturday morning
could even fall at a moderate to heavy clip as it appears a
transient band of fairly strong lower-mid level frontogenesis
will shift across the area beneath a corridor of steepen mid-level
lapse rates. As a result, a quick inch or two of accumulation
will be possible for some during the morning hours, especially for
areas north of I-80. During the afternoon a drier mid-level air
mass will try to shift over the area. Once this arrives we will
see an end to the accumulating snow sometime into the afternoon on
Sunday, with only some light drizzle possible into Sunday
Surface temperatures are expected to warm to near, or even a
degree or two above freezing during the afternoon Sunday, so any
lingering precipitation should be in the form of drizzle into
early Sunday evening. Total snow amounts continue to look the
highest over far northern parts of IL, especially near the WI
state line, where around 2 to 3 inches look possible. Amounts
should tapper off with southward extent over the area.
Following this quick moving weather disturbance, it is looking
like we will see a couple of quiet weather days across the area
Monday and Tuesday of next week as surface high pressure
dominates. Temperatures both days looks to be in the low to mid
30s. By midweek forecast guidance is not in very good agreement
with the evolution of another storm system expected to develop
over the southwest. Some of the 12z guidance suggests this storm
system will largely remain south of the area as a colder airmass
builds southward over our area under another area of high
pressure. However, there remains some guidance and ensembles
supporting some local impacts from this storm system. Given the
high uncertainty during this period, I did not stray from the
general blended guidance, which gives some low chance POPs for
For the 00Z TAFs...
Aviation forecast concerns:
-Chance of flurries/very light snow overnight, and a few flurries
Saturday afternoon. No significant impacts expected to terminals.
-MVFR ceiling potential Saturday.
Weak area of surface low pressure was analyzed over eastern IA
late this afternoon, in association with a weather disturbance
which will track east-southeast through central IL/IN through
Saturday morning. Regional radar mosaic depicts a resulting area
of light precipitation, mainly light snow, across parts of eastern
IA and western IL, which is expected to track largely south of the
terminals. Farther north, weaker forcing and a somewhat moisture
starved thermodynamic profile is expected to produce several
thinner cloud layers tonight and early Saturday, with the
potential mainly for just a few no-impact flurries later tonight.
Little/no accumulation is anticipated for the metro terminals,
with a light dusting at best, with better potential for some minor
accumulations south of the Interstate 80 corridor. Mainly VFR
conditions are expected, though some high-end MVFR ceilings are
possible after midnight.
Modest west winds will diminish quickly this evening as weak high
pressure and the approaching weak surface trough result in a weak
and baggy surface pressure gradient. Winds will turn westerly near
10 kts Saturday behind the departing system. Forecast soundings
would maintain support for an MVFR stratocu deck 2500-3000 ft to
linger Saturday. With the top of the cloud layer colder than -10C,
a few flakes may be produced, though just flurries with no impact
to visibility and no accumulation.
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