Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
122 AM EDT Thu Apr 18 2024

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


...Overview...

A generally progressive flow pattern is expected during the medium
range period with not much expected in terms of significant 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 trough and possible closed
low reach the coast by Wednesday and slowly moves inland.


...Guidance/Predictability Assessment...

The latest 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 deterministic model 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. The 17/12z run of the ECMWF
was much stronger with this, bringing a deep closed low through
southeast Canada and into the Northeast late period, while the GFS
and CMC favored just a weak shortwave. Was not ready to be as
aggressive as the ECMWF so favored the GFS (and CMC), along with
the ensemble means for this system. 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 -- the
southern part likely headed towards the West Coast by next
Wednesday. There is agreement in possible closed low development
off the West Coast but significant differences in the north-south
placement of this low. The CMC at this point seemed an outlier as
it then splits the energy again bringing a closed low into the
Southwest and another one into British Columbia. Favored the
ensemble means pretty heavily for this system, along with smaller
contributions from the GFS and ECMWF.


...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/Carolinas 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 is in place across this
region for the Day 4 Excessive Rainfall Outlook. 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 Wednesday-Thursday.

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