Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/05/20
Area Forecast Discussion
National Weather Service Boston/Norton MA
1026 PM EST Fri Dec 4 2020
A strong storm system will impact the region Saturday into
Saturday night bringing heavy rain and strong winds along the
coast, with potential for heavy wet snow and power outages
across the interior. The storm exits Sunday, then mainly dry
and colder weather Monday and Tuesday with a moderating trend
for the middle of next week.
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
1020 PM Update...
Forecast is largely on track so only made minor changes to
better match the current obs. Our focus tonight will be on
fine tuning the details of tomorrow`s winter storm.
Initial band of light rain along south coast was slowly lifting
north but plenty of dry air at lower levels means it will take
its time making progress. Expect light rain to reach Mass Pike
corridor around 10 or 11 PM but locations farther north may not
see much at all tonight. 22z HRRR seems to have a good handle on
By late evening and especially toward morning intensifying low
pressure across the mid Atlantic states will be lifting
northeast. This increases the forcing for ascent and widespread
rain should overspread most of the region by daybreak. Low temps
will mainly be in the upper 30s to lower 40s...but it will be a
few degrees cooler in the high terrain.
.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
* Heavy Wet Snow with the potential for numerous power outages
and tree damage in the high terrain of central MA
* Uncertainty exists in westward extent of heavy snow/power
outage risk and Ptype issues across the coastal plain
* Strong Winds with Heavy Rain/Street Flooding expected near and
southeast of the Boston to Providence corridor
Low pressure will undergo bombogenesis as it rapidly lifts
northeast from the mid Atlantic and towards the Cape/Islands
over the next 24 hours. The surface low drops over 30 MBs as it
passes to our east at around 980 mb Saturday evening.
If we were dealing with a seasonably cold airmass a blockbuster
snowstorm would be in the cards for the most of the region. In
contrast, the current airmass in place is quite mild for this
time of year with temperatures still in the 50s late this
afternoon. That being said, the rapid intensification and strong
dynamics of this system will allow for soundings to eventually
become isothermal in some locations. As the mid level centers
close off and track across the far southeast New England coast,
very strong omega of 30 to 50 units will support heavy
precipitation. The specific track coupled with very sensitive
thermal profiles will make the difference between heavy rain and
1-2 inch per hour snowfall rates. We will break it down a bit
more below by region.
Lets start off where confidence is highest and that is across
central MA and especially the Worcester Hills. While rain should
be falling Saturday morning, expect very strong dynamics to
result in a change over to heavy wet snow during the mid to late
morning hours. This will first occur in the highest terrain of
central MA and then impact the rest of that region. Given the
evolution of the storm and rapidly developing mid level centers,
expect snowfall rates of 1 to 2 inches per hour once the
transition occurs. Unlike the late October snowstorm...925T will
only drop to between 0 and -2C. The result will be a heavy wet
snow with temp on the order of 32/33 degrees. That is
problematic because we are anticipating 6 to 12 inches of wet
snow in this region with the greatest risk of seeing a foot of
snow in the highest terrain. In fact, we can not rule out the
low risk of 12 to 16 inches of snow. On the flip side of the
coin, we want to be careful given marginal thermal profiles. If
it ends up a little warmer we could up with more rain than
expected and lower snowfall amounts than our current forecast
reflects. But putting it all together, we are quite concerned
for the potential for significant power outages and some downed
trees if the heavy snow amounts are realized. Winter Storm
Warnings have been posted for this region.
Western MA and northern CT...
Despite the event being less than 24 hours away, uncertainty
remains larger than normal for western MA and CT. The 12z model
suite has trended further west and also a bit milder than the
earlier 00z runs. Across western MA and northern CT, the threat
exists of rain changing to heavy wet snow but is dependent on
track. If track shifts slightly back to the east, the bulk of
the heavy snow would remain east of the CT River Valley. The
limited amount of dynamics/intensity would also create Ptype
issues especially in the lower elevations of the CT River
valley. Based on the westward shift, we did opt to include
Winter Storm Watches into Tolland and Windham counties for the
potential of heavy wet snow of 6+ inches and scattered power
outages. The 18z NAM and latest HRRR runs do show some concern
for Hartford county too, so something will have to watch
closely. We also left the remainder of western MA is the Watch
as a small change in track will make a big difference. The 00z
model runs this evening will shed more light on the situation.
Finally, there is an unusual amount of uncertainty across
eastern MA/RI inside 24 hours. While heavy precipitation is
pretty much a lock, the westward shift in the 12z model guidance
has resulted in a somewhat warmer solution. It probably will
take until mid to late afternoon for rain to change to snow
across this region. Still the potential for warning level snows
in metrowest with snow accumulations into the Boston to
Providence late Saturday into Saturday evening as well. Greatest
concern for heavy wet snow and power outage risk will be across
interior northeast MA, where a Winter Storm Watch remains in
place. In fact, we can not rule out heavy snow/warning criteria
in parts of northwest RI if the track shifts a bit east. Winter
Weather Advisories may eventually need to be extended further
Heavy Rain/Strong Winds...
Ptype will mainly be rain near and especially southeast of the
Boston to Providence corridor. In fact...heavy rain on the order
of 2 to 3 inches with some locally higher amounts may result in
pockets of urban and poor drainage street flooding. If 3+ inch
amounts are realized a few mainstem rivers and small streams may
experience minor flooding. Lastly, a high wind warning has been
issued for coastal Essex County as well as coastal Plymouth
County, Cape Cod and the Islands. A very strong pressure
rise/fall couplet in excess of 10 MB in 3 hours will bring NNW
wind gusts of 50 to 60 mph late Sat into Sat evening. We may
have to issue wind advisories further west for a period of 40 to
50 mph wind gusts.
.LONG TERM /SUNDAY THROUGH FRIDAY/...
200 PM Update:
Fairly quiet weather expected through the period. We are
watching another potential cutoff low from the Ohio Valley to
the mid Atlantic states early next week, but with lack of
downstream blocking this time (i.e., no ridging over Greenland)
system is likely to pass well south of New England. Did include
some chance PoPs near South Coast, Cape, and Islands but for
most part expect this system to have little impact on SNE (as of
right now, anyway). Upper flow turns more zonal for mid to late
week which should allow for temperatures to moderate into 40s
to around 50.
.AVIATION /04Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Tonight through Saturday night...Moderate Confidence.
VFR most of the night but expect conditions to lower to MVFR
near south coast overnight and rest of SNE by daybreak Sat,
before lowering to IFR Sat morning areawide.
Rain changes to snow Sat, first near Berkshires and ORH by late
morning (14-16Z) where 1/4-1/2SM likely and snowfall rates of
1-2"/hour most of afternoon. Tougher call from BDL-PVD-BOS but
thinking changeover occurs early afternoon at BDL (18z) then
at BOS/PVD later in afternoon (20-22Z). All rain for
Cape/Islands. Conditions improve quickly Sat evening to VFR,
mainly 02-05Z Sun.
Strongest winds expected later Sat afternoon and evening with
NW winds gusting to 25-35 kt inland and 40-50 kt along the
coast with 50 kt gusts most likely on Cape Cod/Islands.
KBOS TAF...Moderate confidence in the TAF.
KBDL TAF...Moderate confidence in the TAF.
Outlook /Sunday through Wednesday/...
Sunday: VFR. Windy with gusts up to 35 kt.
Sunday Night: VFR. Breezy.
Monday Night through Tuesday Night: VFR. Breezy.
* Storm Warnings Saturday Afternoon and Evening *
Tonight through Saturday night...
A significant Marine event will occur Saturday afternoon and
night as low pressure undergoes bombogenesis and tracks across
the Cape/Islands and moves NE from there. The strongest winds
will be from the N and then NNW as the storm rapidly intensifies
resulting in 45 to 50 knot wind gusts. Strong Gales are in
effect across western sounds, Narr Bay and Boston Harbor with
Storm Warnings for the rest of our waters. 15+ foot seas
expected across our outer most waters.
Outlook /Sunday through Wednesday/...
Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Rough seas up to 14 ft.
Sunday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas.
Monday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Monday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain.
Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of rough seas. Slight chance of rain.
Tuesday Night through Wednesday: Winds less than 25 kt. Local
CT...Winter Storm Watch from 7 AM EST Saturday through Sunday
morning for CTZ003-004.
MA...High Wind Warning from 1 PM Saturday to midnight EST Saturday
night for MAZ007-019-022>024.
Winter Storm Watch from 7 AM EST Saturday through Sunday
morning for MAZ002-003-005-006-008>011-014.
Winter Storm Warning from 7 AM Saturday to 7 AM EST Sunday for
MARINE...Storm Warning from 1 PM Saturday to 3 AM EST Sunday for
Gale Warning from 10 AM Saturday to 4 AM EST Sunday for ANZ230-
Gale Warning from 10 AM Saturday to 7 AM EST Sunday for ANZ235-
Area Forecast Discussion
National Weather Service Lincoln IL
839 PM CST Fri Dec 4 2020
Issued at 318 PM CST Fri Dec 4 2020
Mild and dry weather is on tap tonight through Saturday...before
an upper-level disturbance brings a few sprinkles or snow flurries
to central Illinois on Sunday.
Issued at 839 PM CST Fri Dec 4 2020
Skies are clear and winds are variable. Temps have dropped off
quite a bit already, but probably will not drop much more. Current
overnight low forecast of the middle 20s still looks good, given
current dewpoints in the middle to upper 20s over most of the area
and lower 30s in the southeast. Lower clouds seen on satellite
loops coming off Lake Mich may move into northeastern parts of the
CWA as winds overnight shift to north to northeast, but they
probably wont get here until toward morning. Current forecast
already showing some increasing cloud cover then. Since current
forecast looks good, no update planned at this time.
.SHORT TERM...(This evening through Saturday)
ISSUED AT 318 PM CST Fri Dec 4 2020
21z/3pm surface analysis shows a weak cold front extending from
central Michigan to southern Minnesota. This boundary will drop
southward through central Illinois this evening, resulting in
winds veering from W/SW to N/NE after midnight. With deep-layer
moisture absent, no cloud cover/precip will occur with FROPA. Once
the boundary layer flow becomes northeasterly, lake-enhanced
moisture from Lake Michigan will begin to spill into east-central
Illinois overnight. RAP 900-950mb layer RH field continues to
suggest the NE KILX CWA around Champaign and Danville will become
overcast toward dawn. Meanwhile further west, skies will remain
mostly clear. Low temperatures will drop into the middle 20s. The
lake-enhanced cloud band will linger along/east of I-57 through
Saturday morning before gradually shifting eastward into Indiana
by afternoon. Given E/NE winds, highs will be a few degrees cooler
than today...generally in the lower to middle 40s.
.LONG TERM...(Saturday night through Friday)
ISSUED AT 318 PM CST Fri Dec 4 2020
The next weather system of interest is currently evident on water
vapor imagery over Saskatchewan. As this short-wave trough dives
southeastward, all models are indicating that it will amplify into
a closed 500mb low on Saturday...with the low then tracking across
central Illinois during the day Sunday. Despite a lack of
moisture to work with, think synoptic forcing will be strong
enough to warrant a chance for sprinkles/flurries. Have added
flurries to the forecast Saturday night across the NW CWA,then
everywhere by Sunday morning. With high temperatures reaching the
lower 40s, any lingering light precip will change to sprinkles
A second amplifying short-wave will quickly follow in its
footsteps, carving out a deep long-wave trough across the Great
Lakes/Ohio River Valley by early next week. With the second wave
passing through the region, have continued the chance for flurries
Sunday night into Monday before synoptic subsidence in its wake
brings clearing skies across the board by Monday afternoon. After
that, the deep trough will shift eastward...allowing upper heights
to rise substantially across the Midwest by the middle and end of
next week. As a result, high temperatures will warm into the lower
50s by Wednesday/Thursday.
.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 535 PM CST Fri Dec 4 2020
Weak cold front currently across northern Indiana/Illinois will
sink southward tonight...resulting in winds veering to the N/NE
overnight. Low clouds over Lake Michigan will gradually advect
S/SW tonight and will spill into east-central Illinois toward
dawn. Based on RUC 900-950mb layer RH field, it appears KCMI will
be on the back edge of the lake-enhanced cloud band. Have
therefore maintained a period of MVFR ceilings between 11z and
15z at KCMI, with only SCT low clouds at KBMI/KDEC. Despite
relatively dry airmass, several CAMs suggest patchy fog late
tonight. Since fog formed last night, went with a persistence
forecast and brought visbys down to around 4 miles at all sites