Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
228 PM EDT Thu Apr 18 2024

Valid 12Z Sun Apr 21 2024 - 12Z Thu Apr 25 2024


...Overview...

A reasonably progressive flow pattern is expected during the
medium range period, with not much expected in terms of
significant organized weather hazards. Troughing across the Great
Lakes into the Northeast will be periodically renewed as shortwaves
traverse through the base of the trough, bringing some generally
light precipitation from parts of the Upper Midwest to the East.
Out West, upper ridging early in the week will slide eastward as
the next amplified upper trough/low reach the coast in a week.


...Guidance/Predictability Assessment...

Recent guidance shows fairly good agreement across the CONUS
through much of the period (Sunday-Thursday). There remain some
timing differences with shortwave troughing through the Southeast
Sunday- Monday and again with the next shortwave through the Great
Lakes/Northeast Tuesday-Wednesday. These details create some
uncertainty in the eventual (generally light) precipitation pattern
across the Southeast and Northeast early next week, but a general
guidance compromise for the most part gave a good starting point.
Energy dropping south out of a large closed low over Hudson Bay
early to mid next week also presents a forecast challenge and how
it may or may not interact with the shortwave into the Northeast at
the same time. Opted for another guidance composite. There is also
a lot of uncertainty late period with the evolution of a deep
closed low over the Gulf of Alaska which may split into two
factions with the southern part likely headed towards the West
Coast in about a week. There is some agreement in possible closed
low development off the West Coast, but significant differences in
the north-south placement of this low. Favored the ensemble means
along with smaller contributions from the deterministic models.

...Weather/Hazards Highlights...

Combined shortwave energy across the Tennessee Valley/Southeast
will interact with a front dropping slowly through the Southeast
supporting moderate to locally heavy rainfall from the Central Gulf
Coast into the Southeast on Sunday. Though QPF amounts are not
particularly high, there is some anomalous moisture and instability
present which would support higher rain rates and possible flash
flooding. A Marginal Risk area remains in place across this region
for the WPC Day 4/Sunday Excessive Rainfall Outlook, but have
reduced the areal coverage/inland extent given coastal position of
the nearby/wavy front as per latest WPC/guidance QPF and ensemble
heavier QPF probabilities. The next shortwave through the Northern
Plains-Great Lakes-Northeast will be accompanied by some generally
light to moderate rain next week along the attendant cold front
through the Upper Midwest, Ohio Valley, and Northeast. A flash
flood threat is not anticipated with this system in the absence of
any appreciable moisture anomalies or instability. The next round
of light rain/mountain snow should arrive in the West by next
Tuesday ahead of the amplified trough, with some precipitation
farther inland across the Northern Rockies as well.

The mean upper trough over the East much of the period should
support generally below normal temperatures for most areas east of
the Rockies except Florida Sunday-Monday. Some warm up expected
from the Southern Plains into the Mid-Atlantic around next Tuesday
but the next shortwave into the northern tier should keep
temperatures chilly from the Rockies to the Great Lakes and
eventually into the Northeast Tuesday-Thursday. Initial ridging out
west should support above normal temperatures, but temperatures
look to drop back below normal along much of the West Coast and
inland by next Thursday.

Santorelli/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 (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