Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/20/20
Area Forecast Discussion
National Weather Service La Crosse WI
513 PM CST Sat Dec 19 2020
.SHORT TERM...(This evening through Sunday)
Issued at 231 PM CST Sat Dec 19 2020
Main forecast concerns in the short-term are on cloud trends
and low temperatures tonight.
Stratus remains in place across the entire forecast area this
afternoon. The back edge, located across south central Minnesota
and north central Iowa, was very slowly edging east this afternoon.
RAP forecast soundings suggest the stratus will break up and push
east tonight. However, not expecting to see much drying out of the
stratus layer as there is very little subsidence in place. Thinking
the the stratus will gradually push east on the back side of the
surface ridge as winds in this stratus layer switch around to the
southwest and increase tonight. Clouds will have a huge impact on
temperatures tonight. Where clouds clear lows could fall into the
mid to upper teens. Where clouds remain, lows will stay in the
upper 20s. We should see the clouds clear across southeast Minnesota
and northeast Iowa this evening, then after midnight across western
into central Wisconsin. The daytime hours on Sunday will feature
quiet weather across the area but that changes Sunday night as a
fast moving system sweeps across the area with some light snow. High
temperatures will be pretty pleasant for mid December on Sunday with
highs ranging from the upper 30s over northeast Iowa to the lower
30s across northcentral Wisconsin.
.LONG TERM...(Sunday night through Saturday)
Issued at 231 PM CST Sat Dec 19 2020
Mid-high level GOES water vapor imagery showing the Sunday
weather system now crashing onto the British Columbia coast. Open
cellular convection is seen in a wave energy train upstream of
the coast with GOES 17 cloud drift winds showing a strong zonal
jet of 140-150kts...impressive! This system shifts across the
area Sunday night and Monday with descent QG forcing through the
troposphere and a warm advective band of precipitation. Models
coming more into agreement that snow will fall with this system
north of I-94, with confidence decreasing southward. Some 19.12Z
model solutions /EC, HiRes Window NMM, SREF mean, etc/ splash very
light precipitation southward over most of the area. This is
where confidence is lower right now - just how far south will see
measurable snowfall of a few tenths of an inch of snow. In the
19.00Z ECMWF ensemble, all 50 members snowed in KRST (all <0.5"),
and about 90% in Platteville WI. Light, but a few tenths of an
inch of snow. Generally continued the forecast of 1-2"
accumulations north of I-94 in northcentral WI (agrees well with
19.12Z HREF accumulations), with less than 0.5" elsewhere. But,
have not had the confidence to increase precipitation chances
much south of I-90. It may be that almost all areas see very
light, measurable snow midnight Sunday to noon Monday. Winds
really kick in Monday post-frontally with gusts to 35 mph and
possible some snow/rain showers.
Above-average temperatures are the story through Wednesday, but
the mid-week period is really in question. Model solutions vary
wildly from a Plains blizzard to the northwest to a band of snow
to the south. Inspection of the Canadian/GFS/ECMWF ensemble
families and cluster diagnostics suggests predictability is low
for that period and has not generally increased over the past 18
hours. There certainly could be higher impact weather in the area,
but then again, maybe not. Thus, it seems most prudent to allow
the lower precipitation chance, model blend to ride for now with
&&
.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 513 PM CST Sat Dec 19 2020
Stratus clouds with IFR/MVFR ceilings will hang around this evening
as the cloud deck slowly clears out from west to east tonight.
Some areas of BR will hang around as well with visibilities
possibly dipping into 3-5 SM at times. Not fully confident on the
clearing times, but was a little more pessimistic and slightly
delayed the clearing at KRST. After the clouds clear out around
05z at KRST and 07z at KLSE, VFR conditions are then expected
through the remainder of the TAF period. Winds will generally
remain around 10 knots or less from the west/southwest.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&
$$
SHORT TERM...Wetenkamp
LONG TERM...Baumgardt
AVIATION...Peters
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
337 PM EST Sat Dec 19 2020
.DISCUSSION...
Scattered light precipitation was in place across SE MI earlier this
morning and afternoon as sufficient moisture ahead of an approaching
cold front partially saturated low-levels. Have bumped PoPs down
for this afternoon given the drier radar trends and with hourly
dprog/dt of hi-res showing a drier 0-4 kft layer relative to early
morning runs, but should start to see an uptick in coverage as the
cold front and trough axis, in place across western Michigan, starts
to sweep through the state. Thermal profiles support all rain (some
melting flakes possible at onset) across the Metro region for new
precipitation, given the increasing wbz heights and temperatures now
in the mid to upper 30s. The Tri-cities and Thumb remain slightly
cooler, but an eventual transition to rain is probable as wbz heights
gradually increase later this evening. A dry slot is also projected
to fill in from south to north through the evening, stripping ice
nuclei from the dgz, further supporting an eventual transition to all
liquid. Temperatures are expected to remain just above freezing late
tonight and through the overnight hours, so any additional snow
accumulation will remain limited to elevated and grassy surfaces --
totals under a half-inch, likely confined to the Thumb. Otherwise,
patchy fog development will be possible this evening and overnight
given dew points hold around 32 degrees. Surface cold front/trough
will push across and wash out over SE MI late tonight and overnight
with very subtle backing of low-level winds advecting some Lake
Michigan moisture into the area. This should help support some
drizzle chances and extensive cloud cover during the overnight
period. Overnight temperatures will not drop much as a result,
holding in the lower 30s.
Dry low-level air will gradually fill in behind the front Sunday
morning into the afternoon, greatly diminishing PoP chances. There
will be modest instability for December standards building up during
the late morning and early afternoon, which could help reinvigorate
some spotty light showers, however, the low-level environment is dry
(0-2 km RH below 60 percent) and convective depths are low (below
5kft). Regardless, the HRRR is hinting that some hydrometeor
production could make it down to the surface, so will hold slight
PoPs through the afternoon. Saturation falls well below the moisture-
starved dgz, so any precipitation observed would fall as rain. All
PoP chances to come to an end in the late afternoon and evening as
daylight wanes and dry air continues to fill in. Little change in
the thermal profile to again support highs in the mid to upper 30s
with overnight lows dropping into the 20s.
Strong convergence is noted between the NAEFS and EPS ensemble
forecast system regarding a clipper system that drops from the
northern Plains late Sunday and arrives over Michigan Monday
afternoon into Monday evening. Convergence of this system was also
noted 24 hours ago across ensemble systems. As the low arrives over
Michigan, SE MI will fall on the cyclonically sheared side of a 130
knot jet and will have support of multiple PV waves while westerly
flow helps advect moisture off of Lake Michigan. Given the above,
have bumped up PoP chances to likely (~60-70 percent) in the Monday
afternoon-evening time frame. Temperatures will once again hold in
the mid 30s when precipitation is expected, thus ptype will fall
between rain/snow or all snow. This low pressure will weaken while
approaching Michigan and forcing is not strong, so early accumulation
potential will hold to or below an inch. Last, a large amount of
variance is still observed between ensembles regarding a late week
system expected to impact the Great Lakes region. A strong Pacific
wave will crash onshore late Monday into Tuesday which will amplify
troughing across the southwest. Cyclogenesis will take place as a
result, but large uncertainties exist regarding the potential track
of the surface cyclone -- A wide dispersal is noted for individual
ensemble members. This trough and associated cold front will
eventually impact SE MI later next week, bringing active weather and
colder temperatures around the holiday weekend. Additional
convergence will be needed between models in order to pin down
temperature trends and any snow potential.
&&
.MARINE...
Weak system moving across the central Lakes today as a frontal
boundary stalls out. This will bring weakening southwest flow into
tonight as precipitation lingers over the lakes into Sunday morning.
Another system tracking across northern Ontario will bring moderate
southwest flow again Sunday afternoon with gusts generally under 25
knots. Progressive pattern continues into early next week as an
Alberta clipper is expected to swing across the region Monday and
Monday night. The result will be a surge of colder air and
increasing northwest flow Monday night with gusts approaching 30
knots. The middle of next week also favors unsettled marine
conditions.
&&
.PREV DISCUSSION...
Issued at 1247 PM EST Sat Dec 19 2020
AVIATION...
Low-level moisture continues to stream into Southeast Michigan early
this afternoon with MVFR ceilings filling into the remainder of the
terminal corridor over the next several hours. Ascent ahead of the
inbound low pressure/cold front and sufficient moisture depth
results in additional light precip this afternoon and evening.
Thermal profiles, recent obs, and surface temps above freezing
suggest a rain/snow mix transitioning to light rain later this
afternoon. The exception will be MBS and possibly FNT where light
snow may remain dominant. Midlevel moisture strips out of the area
later this afternoon, bringing into question the presence of ice
nuclei in the cloud layer this evening and especially tonight. As a
result, precip will likely change over to drizzle with fog tonight
as the low-levels remain saturated under the stalled frontal
inversion. IFR ceilings likely develop this evening with a period of
LIFR possible late tonight into early Sunday as the front stalls and
weakens over the area. Conditions then slowly improve during the
daylight hours Sunday.
For DTW...Any light precip should fall as a rain/snow mix early this
afternoon before light rain generally becomes favored by late
afternoon into this evening. Drizzle with fog then becomes most
likely tonight as midlevel dry air moves in over saturated low
levels. MVFR ceilings will spread in from the northwest over the
next 3 to 4 hours with IFR ceilings then becoming likely after 03z.
DTW THRESHOLD PROBABILITIES...
* High in ceilings below 5000 feet this afternoon through Sunday
morning.
* Moderate in precip type as a rain/snow mix this afternoon. High in
precip type being all rain this evening and overnight.
&&
.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&
$$
DISCUSSION...AM
MARINE.......AA
AVIATION.....TF
You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.
Area Forecast Discussion
National Weather Service Nashville TN
539 PM CST Sat Dec 19 2020
.UPDATE...
FOR EVENING DISCUSSION.
&&
.DISCUSSION...
Precip in the form of showers has made its way across the western
half of the mid state this early evening. OHX sounding release is
revealing freezing levels in the 6kt to 7kft range. Latest Hrrr in
agreement with the categorical pops spreading from west to east
overnight, so fcst on track.
Otw, fcst lows are very close to wet bulbs across the area so no
need to adjust our mins. Fcst appears to be on track so no
changes.
&&
.AVIATION...
00Z TAF DISCUSSION.
Rain continues to move in from the west, and cigs will continue to
drop this evening and overnight from MVFR to IFR by sunrise. LIFR
cigs will also be possible, especially from around 12Z through 16Z
or so except for KCSV where LIFR/IFR cigs will be in place for
most of the day Sunday. Chances for rain will diminish from west
to east starting around 06Z at KCKV, and fog may develop behind
the rain by 12Z at KCKV. Cigs should slowly improve during the day
for KCKV/KBNA/KMQY and should be VFR by 00Z Monday at those
terminals. Winds will generally be light and variable.
&&
.OHX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
DISCUSSION......21
AVIATION........Barnwell