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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1848Z Feb 26, 2024)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
148 PM EST Mon Feb 26 2024

Valid 12Z Thu Feb 29 2024 - 12Z Mon Mar 04 2024

...Heavy Precipitation Pattern Emerging Again for the West...

...Guidance/Predictability Assessment...

Model guidance remains fairly well clustered after initially
struggling to find a consensus with a split-stream flow pattern
over the CONUS earlier this week. Following the departure of a
weakening southern stream shortwave moving northeast from northern
Mexico across the Southeast and off the coast of the Mid-Atlantic,
attention then turns to to the West as energy upstream over the
northeastern Pacific begins to slowly dig southward along the West
Coast. This will allow for a multi-day influx of Pacific Moisture
over the West and yet another Atmospheric River event. The pattern
over the CONUS will become more amplified as this deep longwave
trough shifts eastward over the West and towards the central U.S.
this weekend into early next week, with stronger ridging/higher
heights over the East. There remains the typical increasing
uncertainty with time on some of the details of individual
shortwave energies/frontal systems within the flow, specifically
with respect to a system tracking along the Canadian border across
the north-central U.S. this weekend, and the intensity and
northward trend in location with a potential embedded closed
upper-level low/deepening surface cyclone pushing northeastward
across the Plains/Midwest next week as the longwave trough shifts
eastward. However, the overall pattern evolution looks to remain
fairly consistent with little run-to-run variation through
yesterday's 00Z and today's updated 12Z forecasts. Given the
guidance is well clustered, a multi-model blend is used for the
updated WPC forecast for the early to mid-forecast period, with a
contribution from the ECens/GEFS means added for the later period
to account for some increasing uncertainty with the smaller-scale
details. The consistency in the guidance along with a similar
guidance blend to the prior WPC forecast led to minimal changes
overall with this update.

...Overview and Weather/Hazards Highlights...

A leading/main southern stream upper trough with origins over the
East Pacific will eject eastward from northern Mexico and traverse
The South/Southeast Thursday and Friday as a surface low/frontal
system takes position over the Gulf of Mexico. Moisture return
from the southern Gulf and easterly fetch from the Atlantic are
expected to fuel an area with rain/convective chances. Locally
heavy downpours may focus over the Central Gulf Coast, with
additional but more uncertain potential for a secondary maxima
inland over the Southeast/southern Appalachians. The area will be
monitored for any runoff issues, but no ERO threat area has been
issued at this time given relatively high Flash Flood Guidance
values. Rain/showers will linger over the Southeast this weekend
with more, but weaker impulses in a similar pattern.

Upstream, there continues to be a very strong guidance signal that
an amplified upper trough digging into the eastern Pacific later
this week will shift over the West this weekend. This will
increasingly support chances for a moderate to heavier rain focus
into the Pacific Northwest mid-later week that along with an
associated/moderate long fetch moisture plume/Atmospheric River
will shift steadily down the coast through southern California
later week into the weekend to monitor. The WPC Day 4/Thursday and
Day 5/Friday Excessive Rainfall Outlooks (EROs) offer Marginal
Risk areas for coastal Oregon through Northern California. There
is also a significant threat for high winds and very heavy snow to
spread inland across the West, especially for favored
terrain/mountains as highlighted in the WPC Winter Weather
Outlook. Confidence continues to increase in the risk for moderate
to extremely disruptive high snowfall rates/blowing snow,
particularly for the Sierra Nevada. Additionally, some valley
snowfalls may develop more over time as snow levels decrease with
upper system and wavy frontal translation. This translation should
also favor development of an emerging precipitation shield over
the cooling Rockies/Plains by Sunday/next Monday. Widespread
showers/thunderstorms look to range across portions of the
Plains/Mississippi Valley which may include some severe weather
potential, as well as the chance for some accumulating snows on
the northwest periphery of activity across the Northern Plains to
Upper Midwest.

Upper trough amplification and inland march by the weekend will
cool temperatures over much of the West, leading to maximum values
of as much as 10-15 degrees below normal. Downstream, expect much
above normal pre-frontal warmth later week over the north-central
U.S. that will spread to much of the central and eastern U.S. this
weekend/early next week downstream of the main upper trough/lead
frontal system working over the West. Spring maximum temperatures
ranging upwards to 20-30 degrees above normal will spread from the
Midwest toward the Northeast to offer potential for some record
values.

Putnam/Schichtel


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