Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1205 PM EDT Tue Apr 30 2019
Valid 12Z Fri May 03 2019 - 12Z Tue May 07 2019
...Heavy Rain/Flooding Threat Fri/Sat for the Southern
Plains/Lower Mississippi Valley...
...Overview...
Upper ridging over the northeastern Pacific will remain stationary
during the medium range period which will drive a split flow
across the Lower 48. Features in the forecast will be the northern
stream out of NW Canada and an incoming closed low in the southern
stream into California around Sunday. Ridging off the Southeast
coast will wobble in place allowing for wavy fronts along the east
coast as systems exit into the Atlantic.
...Guidance/Predictability Assessment...
In the southern stream (off the west coast), the GFS/Canadian and
their ensemble means were quicker than the ECMWF/ECMWF ensemble
mean to move the upper low into California either late Saturday or
early Monday as the timing difference exceeded 36 hrs between the
quicker/slower solutions. 00Z UKMET was about 2/3rds the way to
the slower ECMWF from the GFS and this was preferred given: 1) the
separation from either an upstream kicker or the northern stream
in the Pac NW/Great Basin, 2) tendency of the GFS to be too quick
to bring closed lows eastward in weak patterns and 3) continuity
which favored the slower side of the ensembles.
Over Texas, tail end of a front will linger on Friday and then
slowly move southeastward on Saturday as a wave moves along the
boundary into the Southeast then off the mid-Atlantic coast
Sunday. This will provide a focus for the heavy rain potential
(partly into the short range period) along and east of I-35.
In the northern stream, consensus approach was maintained which
resulted in only a few changes to frontal timing/placement. System
moving out of the Dakotas on Saturday will lift northeastward
toward Hudson Bay but the ECMWF/UKMET became much quicker than
most of the ensembles whereas the GFS/Canadian were more in line
with the ECMWF ensemble mean. Preferred to rely mostly on the
ECMWF ensemble mean and continuity by Sun-Tue via a larger blend.
Confidence decreased by next Mon/Tue in the latitude of the front
in the east given a flatter flow aloft and spread in ensembles.
Western upper low is forecast to push past the deserts around the
middle of next week per current timing.
...Weather Highlights/Threats...
The front over Texas will tap pooled moisture over its surface
boundary aligned parallel to the upper flow Friday which supports
a threat of heavy rain/thunderstorms and cell training/locally
enhanced runoff issues as embedded hard-to-time weaker shortwaves
move through. More defined shortwave should carry the front
southeastward Saturday and move the rain threat eastward, focused
mostly along the central Gulf coast Sunday. Rain threat remains
into next week as the front lingers along the Gulf coast. This
will generally keep temperatures closer to or just below typical
values for early May.
Pacific closed low will move into CA early next week which will
spread scattered Southwest-to-Rockies showers/convection as height
falls move eastward. Moisture anomalies may only be +1 to +1.5
sigma from normal per the guidance and other ingredients may be
marginal at best, but expect at least isolate showers/storms
especially over the terrain from the Sierra across the Great Basin
next week. Temperatures will trend toward cooler than average as
cloud cover and precipitation increase.
In the east, modest rain will accompany/precede frontal passages
with higher amounts south of about 40N in the warmer airmass.
Northern system will bring more focused precipitation (some snow
in the west at higher elevations) as the lead cold front
dissipates Saturday to be overtaken by a trailing system moving
into the Great Lakes Sunday. Baroclinic zone along the front may
periodically focus convection contingent upon mesoscale
enhancements. Temperatures will be above average ahead of and
south of frontal boundaries by about 5-10 degrees F.
Fracasso
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards 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