Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 PM EST Fri Dec 08 2023

Valid 12Z Mon Dec 11 2023 - 12Z Fri Dec 15 2023

...Overview...

Deepening surface low pressure crossing New England on Monday will
bring the potential for heavy rain and snow to the region before
the system exits by Tuesday. After a bout of light to moderate
precipitation in the Northwest on Monday as well, much of the
lower 48 will trend drier by Tuesday. By midweek and beyond
though, return flow from the Gulf of Mexico streaming into the
southern High Plains ahead of a Four Corners upper low should lead
to increasing precipitation chances there. Moist easterly flow
could spread convection into the Florida Peninsula with a tendency
toward higher rainfall totals late in the week.


...Guidance Evaluation/Predictability Assessment...

The models and ensembles agree with the larger scale aspects of
the pattern. A leading amplified trough should lift away from the
East while a more shallow trough centered over south-central
Canada as of Monday ultimately crosses the northeastern quadrant
of the lower 48 and eastern Canada. Meanwhile consensus is
maintaining the idea of incoming Pacific energy consolidating into
a closed low over the Great Basin/Southwest/Four Corners area by
midweek while an upper ridge builds into the Northwest and then
the central U.S. around the upper low. There is also good
agreement with eastern Pacific trough amplification mid-late week
with a ridge rebuilding near the West Coast. However, there is
considerable spread on how a shortwave nearing the Pacific
Northwest around Wednesday may split as it nears the coast, with
downstream impacts on the western upper low. Upper level/surface
specifics across the Gulf of Mexico and Florida also become a
question mark by late next week, with implications for details of
the general wetter trend signaled in the guidance.

Focusing first on the spread that develops over the western half
of the country after midweek, there is still a relative majority
of guidance that is not as strong as recent ECMWF runs with
incoming midweek energy which that model (at least in the 00Z run)
uses to form a separate closed low and eject the leading midweek
western upper low with flatter trough to the south. The 00Z CMC
fell into the ECMWF camp as well but the new 12Z run has trended
more sheared with its energy. Not surprisingly, a meaningful
number of ECMWF ensembles reflect some aspects of the operational
run but the resulting mean does tone down the more extreme aspects
of the operational run. The majority of GEFS/CMC ensembles side
with recent GFS runs in showing minimal influence from the
incoming energy, holding onto a strong western ridge by Friday
while a well-defined trough with at least an implied upper low
reaches the High Plains. Over the past couple days the 00Z runs of
ECMWF-initialized machine learning models have strongly favored
some variation of the GFS/GEFS/CMCens cluster. Given this majority
scenario and the multi-day mean perspective favoring mean ridging
over the Northwest (which tends toward low predictability for
whatever energy may try to embed within it), preference maintains
continuity by using a late-period model/ensemble mean blend that
minimizes influence of the operational ECMWF.

Meanwhile, southern tier/Gulf of Mexico detail differences become
noticeable late in the period as well. In varying ways the latest
GFS/ECMWF runs are somewhat stronger with approaching shortwave
energy by Friday, leading to more pronounced surface waviness over
the southeastern Gulf of Mexico and heavier rainfall extending
into Florida. Thus far the CMC and GEFS/ECens/CMCens means keep
the surface pattern suppressed over and near the Gulf, so for the
time being will await a more pronounced clustering before
adjusting toward the ECMWF/GFS details. The blend favored for the
late-period western pattern reflects this preferences as well.

A composite of 00Z/06Z models (more GFS/ECMWF weight relative to
the UKMET/CMC) provided a good starting point for the first half
of the period, with good clustering for the New England system on
Monday and other meaningful Monday-Wednesday features.


...Weather/Hazards Highlights...

Precipitation will be ongoing across the Northeast on Monday. Much
warmer than average temperatures are possible across Maine in
particular early in the day, which along with a potential band of
heavy rain (within an axis of precipitable water values at least
2-3 standard deviations above normal) could cause quickly melting
snow for possible flooding there. The Day 4/Monday Excessive
Rainfall Outlook maintains a Marginal Risk for this potential, and
river flooding is also a threat. On the backside of the low,
probabilities for notable snow are still gradually increasing
especially in the higher elevations of the interior Northeast on
Monday. Some brisk to strong winds are also possible from the
south ahead of the cold front and from the west behind it.
Precipitation should clear out by Monday night, perhaps aside from
lingering light snow to the lee of Lake Ontario and favored
terrain near the Canadian border.

Some light to locally moderate precipitation is likely across the
northern Rockies on Monday with potential for some lingering
enhanced snow totals. After that, precipitation will be at a
minimum across the lower 48 on Tuesday, but chances for
precipitation should increase on Wednesday-Thursday across the
southern High Plains as an upper low settles over the
Southwest/Four Corners and moisture interacts with a front. Some
of this precipitation may be snow, with typical elevation/coverage
uncertainty. Increasing spread for specifics of western-central
U.S. flow aloft by the end of the week lowers confidence in how
precipitation may progress eastward over the Plains on Friday. A
weakening front may bring some light precipitation to the Pacific
Northwest around midweek or so, with only light/scattered activity
extending farther inland if at all. Expect Florida to see more
rainfall with time as easterly flow sets up to the south of strong
eastern U.S. high pressure and deep moisture increases. There are
some signals for heavier rainfall by the end of the week but
details will depend on low-predictability impulses in southern
stream flow and whether they are strong enough to support a
surface feature.

As noted above, temperatures will be well above average in the
Northeast on Monday. Morning lows could be near or above record
high minimum temperatures for the date but colder air moving in
later in the day could make it difficult to hold such values for
the calendar day. Coolest anomalies over the East early in the
week will tend to be south of the Ohio River, with the Gulf
Coast/Florida seeing a day or two of temperatures 10F or so below
normal. Otherwise the East should see generally near normal
temperatures, perhaps with moderately above normal lows over the
Northeast most days and above normal highs over all but the South
by Friday. Farther west, the most persistent above average
temperatures will be across the north-central U.S. (other than a
brief cooler spell on Tuesday) as upper ridging builds. The
greatest anomalies look to be around Thursday-Friday for the
Dakotas into the Upper Midwest with readings up to 15-20F above
normal. Most of the West will tend to see near to somewhat above
normal temperatures. Clouds/precipitation over and near the
southern High Plains mid-late week will support a fairly narrow
temperature range (above normal lows, below normal daytime highs).


Rausch/Tate


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key
Messages are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw