Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
157 AM EDT Sun Apr 21 2019
Valid 12Z Wed Apr 24 2019 - 12Z Sun Apr 28 2019
...Overview and Guidance Preferences...
Models continue to struggle with interaction between a more
progressive northern stream and less progressive southern stream
through much of the extended period. Timing and amplitude
differences of features within each stream are compounded by
differences in the potential interaction (or lack thereof) between
streams. As a result, forecast confidence/predictability continues
to quickly decrease by the latter portion of the forecast period.
Southern stream shortwave energy is forecast by all guidance to
cross the Southern Plains Wed/Wed night and move into the lower
Mississippi Valley on Thu, with perhaps a closed center developing
at 500 hPa (although this continues to vary by model and from run
to run). In general, timing of the system has continued to drift
toward a middle ground, and a consensus blend of deterministic
guidance should now represent the system reasonably well at least
through day 4 (Thu). Timing differences continue to grow from Fri
onward, generally related to interaction with northern stream
energy crossing the Great Lakes. Preferred to gradually lean more
toward ensemble timing of this system starting on Fri and through
next weekend, as most solutions show a larger degree of phasing
occurring as the system passes New England.
Farther west, models show general agreement that a rather
amplified northern stream shortwave should approach the Pacific
Northwest on Fri, moving inland Fri night-Sat, lowering heights
and pushing a cold front inland. Solutions vary widely as to what
will occur with the southern stream, however. Models have
oscillated between a cutoff upper low approaching California late
Sat into Sun, or a more elongated and progressive feature that
quickly moves inland. At this time, prefer to lean heavily toward
ensemble means given the spread and inconsistency of solutions
with the southern stream feature.
Given these considerations, the forecast during days 3-4 (Wed-Thu)
was based heavily on a deterministic multi-model blend, including
the 12Z ECMWF/UKMET and 18Z GFS. By days 5-7 (Fri-Sun), weighting
of ECENS and NAEFS ensemble means was significantly increased to
account for increasing spread/uncertainty.
...Weather Highlights/Threats...
The system crossing the Southern Plains and lower Mississippi
Valley Wed into Thu is expected to produce widespread showers and
thunderstorms, with models showing a strong signal for heavy
rainfall from portions of central Texas east toward the
Mississippi River. Severe thunderstorms are also possible. Please
see convective outlooks issued by the Storm Prediction Center for
further details on the severe weather threat. Scattered to
numerous showers and thunderstorms are expected to accompany the
system eastward into the Southeast as well as the Tennessee/Ohio
Valleys by late Thu into Fri, but any signal for heavy rainfall is
much less clear in the guidance given increasing forward speed of
the system and inability of models to resolve convective details
at that time scale. Nonetheless, would expect continued potential
for at least locally heavy rainfall. Farther west, the energetic
upper-level trough entering the northwestern U.S. late in the week
is expected to produce rain and mountain snow from the Northwest
to the northern Rockies. As the energy moves into the central U.S.
by next weekend, rain and possibly even snow may spread into areas
from the Northern Plains to the Upper Midwest/Great Lakes.
Above average temperatures are expected through much of the period
across portions of the western U.S. High temperatures are forecast
to be 10 to 20 deg F above average from California to the Great
Basin Wed-Thu, with these temperature anomalies spread into the
Rockies and portions of the High Plains by Fri-Sat as cooler
temperatures overspread the Northwest/northern Rockies in
association with the arriving upper trough/cold front.
Ryan
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