Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
238 AM EDT Tue Apr 20 2021
Valid 12Z Fri Apr 23 2021 - 12Z Tue Apr 27 2021
...Heavy Rainfall/Flooding Threat across the South Friday into
this Weekend...
...Overview...
Blocky upper ridging forecast over Alaska and Greenland at high
latitudes and across the Greater Antilles at lower latitudes
favors upper troughing over much of the CONUS during the medium
range. One system will push into the West Coast this weekend as
another moves through the Southeast and turns the corner up along
the coast as a nor'easter into Monday. Ridging will build in
between these two systems next week across the Plains ahead of the
advancing western system. The pattern is rather chilly for late
April with precipitation chances focused near each frontal system
in the West, South, and East.
...Guidance/Predictability Assessment and Preferences...
Through the 12Z/18Z model cycle, a blended deterministic solution
sufficed for the first few days of the forecast. Finally the
models have mostly converged on the system timing out of the
West/Southwest, mostly in line toward the ECMWF-thinking. Trend
has been a bit quicker through the lower Mississippi Valley, but
was hesitant to jump as quick as the ECMWF at this point. A
middle-ground solution fit well. As the system lifts toward the
coastal Mid-Atlantic/Northeast, 18Z GFS and 12Z ECMWF formed a
reasonable pairing near their ensemble means and each other
(Canadian too weak and farther south), wrapping up another
occluded system near Maine. Back to the West, trend has been only
a bit quicker, perhaps still within the noise, with much better
ensemble overlap than in the previous several days. Gave extra
weighting to the ECMWF ensemble mean as it has been most stalwart
in this wintry/blocky flow pattern, though the GEFS mean depicted
a plausible solution as well. Utilized the 00Z/01Z NBM for most of
the gridded fields with some added ECMWF ensemble mean influence.
...Weather/Hazards Highlights...
Rainfall will expand along and ahead of a warm front across the
lower Mississippi Valley on Friday, aided by Gulf of Mexico
moisture and instability. Locally heavy rainfall is likely into
the Southeast through late Saturday as the area of low pressure
moves eastward. Runoff and flooding issues are quite possible,
especially in areas that have received much above normal rainfall
this month. The system is then forecast to lift up the East Coast
as trailing northern stream energy from the Great Lakes merges
into the coastal system. Enhanced rainfall is possible into New
England with windy conditions region-wide as the low deepens.
In the West, an amplifying Pacific upper trough/low and surface
front will work into the West Coast over the weekend into early
next week, eventually bringing a lower elevation rain/mountain
snow focus into CA (mainly the Sierra, but even down into the
higher elevations of Southern California). Rainfall does not
appear to be more than light to perhaps modest along the coast.
Once the system moves to the Plains by Tuesday, potential will
increase for convectively-driven rainfall.
A chilly air mass Friday over the East (but especially eastern
Montana) will moderate this weekend. An area of above normal
temperatures over parts of the West/Great Basin ahead of the upper
trough/low will shift eastward across the Rockies and into the
Plains by next week. Cooler than normal temperatures will spread
into the West Mon-Tue as readings may be 5-15 degrees colder than
normal.
Fracasso
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, winter weather outlook probabilities
and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml