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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0659Z Apr 08, 2024)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Mon Apr 8 2024

Valid 12Z Thu Apr 11 2024 - 12Z Mon Apr 15 2024


***Areas of heavy rain likely to reach the East Coast by Thursday
 with strong storms possible over the Southeast***


...General Overview...

Consistent with recent days, latest guidance suggests that a
short-range Southwest/southern Plains upper low will be in the
process of phasing with digging northern stream flow as of the
start of the period early Thursday. From that time, Ohio Valley
surface low pressure should lift into eastern Canada as upper
energy consolidates into a Great Lakes/eastern Canada upper low
while the trailing trough continues through the East
Coast/Northeast. This system will spread a broad area of rainfall,
some of it heavy, along the East Coast Thursday into early Friday
before a drier trend for the weekend. A trailing upper ridge
progressing from the West into the Plains/Mississippi Valley will
support similar movement of an area of well above normal
temperatures. Behind this ridge, guidance continues to show
pronounced trending for what a growing consensus shows will be an
upper low that should drop south from just south of Alaska to
offshore California and then track into the West, spreading
precipitation ahead of its path.

...Guidance/Predictability Assessment...

The forecast of the system tracking northeast from the Ohio Valley
through the Great Lakes into eastern Canada remains consistent for
the most part. ECMWF runs have been the most stable recently with
an intermediate surface low position over western
Kentucky/Tennessee as of early Thursday, while persistently faster
GFS runs have started to nudge slower toward the ECMWF in recent
runs. Some CMC runs including the new 00Z version start out
slow/south but catch up soon thereafter. Thursday preference
continues to place greater weight on the ECMWF versus other models
while a more even weight looks reasonable Friday onward.

Regarding the East Pacific into western U.S. evolution, the past
couple days of ECMWF-initialized machine learning models (MLs) have
generally been providing the lead on the rapid trends toward slower
and deeper troughing, and now a pronounced signal for an upper low
to drop southward to a position offshore California and then
progress inland. Most of the 12Z MLs supported tilting the forecast
at least somewhat toward the 18Z GFS/12Z CMC that tracked the upper
low more offshore California by Saturday, versus the farther east
12Z ECMWF/UKMET. Later in the period the general trough axis in the
18Z GEFS/12Z ECens means also supported slower progression than the
12Z ECMWF, in favor of the GFS/CMC bringing the upper low only
to Arizona or the Four Corners region by early Monday. The 00Z
UKMET and ECMWF have in fact trended more offshore with the upper
low and the new ECMWF is aligned better with the GFS/CMC and
ensemble means by next Monday.

Finally, there is indication of some upper troughing reaching the
Northwest by early next week. The 18Z GFS appeared a little extreme
with its upper low reaching the Northwest, while the 00Z GFS/CMC
hold the upper low farther northwest. Ensemble means and the ECMWF
have flatter flow, while the MLs have varying details but generally
support some degree of troughing (but not an 18Z GFS upper low)

Guidance preferences through the 12Z/18Z cycles led to starting
with more ECMWF weight than other models for Thursday but then
trending to slightly more total GFS/CMC weight versus the ECMWF
thereafter, with some GEFS/ECens input after Saturday.

...Weather/Hazards Highlights...

With continued detail differences for QPF details, there is still
reasonable consensus that low pressure lifting northeastward from
the Ohio Valley and the trailing cold front will spread a broad
area of enhanced moisture northward across the East on Thursday.
The Day 4 Excessive Rainfall Outlook valid Thursday-Thursday night
maintains continuity from the prior Day 5 outlook for the Slight
Risk area over parts of the parts of the northern Mid-Atlantic into
central/south-central Appalachians. Aside from the drier GFS/GEFS
scenario, this region shows the best guidance overlap and
continuity for relatively higher rainfall totals as well as higher
antecedent soil moisture and current stream flows. Other areas
within the large surrounding Marginal Risk still have varying
degrees of sensitivity and mixed messages from the guidance
regarding location/timing of enhanced rainfall. By Friday most
guidance still shows anomalous moisture over the Northeast
along/ahead of the cold front sweeping through the region. The
initial proposal is for a Marginal Risk area across northern parts
of the region. Guidance remains mixed for explicit QPF, but the
overall pattern appears supportive of some enhanced rain rates and
this region should be more sensitive given current snow cover
likely to melting over the coming days. Colder air behind the front
could change some precipitation to snow by Friday night, with some
areas of rain/snow possibly extending into Saturday. This storm
system may also produce brisk to strong winds over some areas. The
Storm Prediction Center is monitoring the potential for severe
weather with the convection forecast to track across the Southeast
on Thursday. Check their latest outlooks for more information.

Continued pronounced trending for the eastern Pacific into western
U.S. pattern, specifically the closed upper low now expected to
drop to a position offshore California and then track inland, has
led to a lighter trend for what rain/mountain snow may reach the
Northwest/northern Rockies late this week but a brief increase of
precipitation focus for California by the weekend, with some
moisture possibly coming into the Southwest thereafter. Upper
troughing that may reach the Northwest could produce a little more
precipitation by late weekend/early next week. Meanwhile the latest
consensus West Coast evolution allows for faster progression
across the northern tier, potentially bringing a frontal system and
some rainfall to the Great Lakes/Ohio Valley/northern Mid-
Atlantic by Sunday or Monday.

An area of above normal temperatures will move from the West into
the Plains Thursday into the weekend, corresponding to upper ridge
progression. Expect daytime highs up to 10-20F above normal over
the West during Thursday-Friday, increasing to 15-25F above normal
over the northern-central Plains and vicinity during the weekend.
Some of this warmth with lesser anomalies will likely spread into
the East by Sunday-Monday. Meanwhile, ongoing changes for the
East Pacific system moving into the West have led to below normal
highs becoming more confined to California and the Southwest during
the weekend while some degree of upper troughing reaching the
Northwest may bring slightly below normal readings to that region
by Sunday or Monday. The cold front pushing through the East late
this week will bring unseasonably warm overnight lows in the warm
sector followed by a brief cooler period (especially over the
Appalachians on Friday).

Rausch


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 (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

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