Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Tue Apr 16 2024

Valid 12Z Fri Apr 19 2024 - 12Z Tue Apr 23 2024


...Overview...

The pattern during the medium range period should transition
towards more progressive flow after the start of the period on
Friday, as an Omega block over the northeast Pacific into western
North America and the northern U.S. tier begins to break down. A
cold front through the East late this week will be accompanied by
some showers and thunderstorms, with the Northeast drying out
thereafter. From the Southern Plains into the Southeast, a slow
moving cold front and shortwave energy will increase the heavy
rainfall potential, particularly across much of Texas on Saturday.
Into next week, fairly low amplitude troughing will be present
across both the West and East coasts, with upper ridging in
between, and generally fairly quiet conditions across the CONUS.


...Guidance/Predictability Assessment...

Latest guidance shows good agreement Friday and Saturday as an
upper low shifts into southeast Canada, bringing low amplitude
troughing to the northeastern U.S., while energy out of a deep low
in the Gulf of Alaska shifts through the eastern Pacific toward the
Northwest. By Sunday, there remains quite a bit of variability on
the strength and timing of the western energy as it moves inland
and towards the Northern Plains. GFS runs have consistently been on
the weaker side with this shortwave while CMC and ECMWF runs are a
bit deeper and slower (though the 00Z and new 12Z ECMWF runs are
faster than the slow outlier of the 12Z 4/15 run). Considering
typical model biases, generally preferred the non- NCEP models
though balanced somewhat with the GEFS mean for a middle ground.
After this, there are some differences in the details of energy
rounding the base of the Eastern U.S. trough, and even greater
uncertainty by Monday-Tuesday with another trough moving towards
the West Coast.

The WPC forecast today used a blend of the deterministic models
for Days 3-4. Gradually transitioned toward using more GEFS and
NAEFS ensemble means through the period to 60 percent by day 7. The
EC ensemble mean could not be used due to ongoing data issues but
it looked to be in good alignment. This maintained good continuity
with the overnight forecast.


...Weather/Hazards Highlights...

A cold front moving through the Northeast Friday-Saturday will be
accompanied by some generally light precipitation and storms, with
the southern portion slower to move or possibly stalling briefly
into the weekend across the Southern Plains-Southeast region.
Anomalous moisture and instability present in the vicinity of the
wavy front favors a low-end Marginal Risk area on Friday for parts
of the southern Plains and Mississippi Valley. This overall area
may have some potential for storm training with higher rain rate
potential, though guidance signals do not show great coherence for
details yet. Also most of this area has been rather dry as of late,
which should inhibit the overall flash flood threat, so the
Marginal is pretty borderline. Favorable flow into the Central
Rockies should work to enhance precipitation there too, with some
snow in the higher elevations at least. By the weekend, weak
shortwave energy reaching into the Southern Plains should help to
further enhance precipitation across Texas and vicinity on Saturday
with increasing heavy rain potential. Model guidance overall has
trended toward a southwest shift with the heavy rain, and thus have
shifted/expanded the Slight Risk toward this trend. The area
covers west-central Texas eastward toward Texarkana with a broader
surrounding Marginal. The front should progress and lay flatter
across the Southeast on Sunday and bring some rain there before
shifting mostly offshore for the workweek. Meanwhile some light to
locally moderate precipitation could come into the Northwest on
Saturday and shift across the northern tier early next week but
with considerable uncertainty.

By Friday and beyond, much of the Northeast should gradually cool
behind a cold front. Meanwhile, the upper trough over the northern
tier states and eventually into the East will support below normal
temperatures (with greater anomalies for highs than lows) for most
east of the Rockies but Florida by next weekend. Above normal
temperatures across the Southwest should gradually expand northward
underneath a building Western U.S. ridge and progress eastward
into the Rockies early next week. Northern parts of the West may
stay near normal due to the upper shortwave/cold front crossing the
region this weekend.


Tate/Santorelli


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