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FXUS02 KWBC 090656

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EST Tue Mar 09 2021

Valid 12Z Fri Mar 12 2021 - 12Z Tue Mar 16 2021

...Late week/weekend heavy rain threat from the south-central
Plains to the Lower Ohio Valley...
...Late week/weekend heavy snow threat over the central Rockies
and High Plains...

...Overview and Guidance/Predictability Assessment...

A robust upper trough with embedded closed low will track slowly
through the Southwest and into the Plains and be a prominent focus
of the forecast late this week into the start of next week. This
system will support areas of enhanced rain and higher elevation
snow over the southern Great Basin/Southwest and a favorable
system track, deformation, and several days of fairly strong
upslope flow suggest increasing potential for a significant heavy
snow event with focus over the central Rockies and High Plains.
This system offers good forecast confidence and continuity into
the weekend.

However, rapidly increasing model/ensemble spread for evolution of
energy within an upstream Pacific trough lowers confidence in how
much upper troughing (and associated precipitation
coverage/amounts) may ultimately reach the West late in the
period. This issue in turn begins to affect the evolution of the
Southwest system once it reaches the Plains. Guidance is all over
the place with ejection and lifting of this main feature over the
central U.S. and continuity is not well maintained days 6-7.
Model/ensemble spread and recent behavior leads to rather low
confidence for how flow with the approaching Pacific trough may
separate and thus what the resulting shortwave (possibly upper
low) looks like as it heads into the West late in the period.
Given the uncertainty, the WPC favored blend reflects only a
portion of the deepening trend for the incoming Pacific trough.
This still leads to a northward lift for the Plains system into
next week. However, the WPC solution leaned increasingly toward
the ECENS mean that does not bring the Plains system as quickly
northward as the 12/18 GFS or the 12 UTC ECMWF. The 00 UTC GFS has
trended a bit slower, but the 00 UTC ECMWF is now a lot slower.

Farther east a departing Upper Great Lakes wave will anchor a cold
front that crosses the eastern U.S. (with support from amplified
northern stream upper trough passage) but settles into the central
Plains/Southeast. Interaction of this front with Gulf moisture and
instability, and then reinforcement as the Southwest system tracks
into the Plains, will likely promote a multi-day threat for
locally enhanced rainfall and training convection.

Overall, the WPC medium range product suite was primarily derived
from a composite blend of best clustered guidance from the 18 UTC
GEFS mean, 12 UTC ECMWF/ECMWF ensemble mean and the 01 UTC
National Blend of Models Friday into Sunday before shifting blend
emphasis increasingly toward the 12 UTC ECMWF ensemble mean by
early next week.

...Weather/Threats Highlights...

The system tracking through the Southwest and into the Plains
should produce the heaviest snow over the central Rockies and High
Plains toward the end of the week through the weekend, after first
producing areas of locally enhanced to possibly heavy rain/higher
elevation snow over the Great Basin/Southwest late this week.
Typical guidance errors 4-5 days out in time suggest some ongoing
uncertainty for exact track of the system and location of highest
totals but in general the guidance signals are getting stronger
for the potential of a significant central Rockies/High Plains
heavy snow event. Some snow may extend farther northeastward over
the north-central U.S. and Midwest from late weekend into early
next week depending on system evolution and track. Confidence for
this latter part of the forecast is much lower though.

Model/ensemble consensus is highlighting an area from the
south-central Plains into the Lower Ohio Valley for highest
five-day rainfall totals during the period. Late this week into
the weekend there will be a persistent flow of Gulf moisture into
a front that stalls over the south-central Plains/Mississippi
Valley while the rest of the front crosses the East. Arrival of
height falls aloft from the Southwest and associated surface
system will provide reinforcement for heavy rainfall potential
later in the weekend/early next week. Meanwhile the Storm
Prediction Center is monitoring the threat for severe
thunderstorms over and a little east of the Southern Plains. Check
the latest outlooks for more information as severe weather details
come into clearer focus.

Expect the Northwest to see an increase of rain/mountain snow and
a cooling trend by the weekend into early next week with the
arrival of an upper trough and cold front. The details of timing,
coverage, and amounts are uncertain at this time given guidance
spread for specifics of the upper trough.


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:

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

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