Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
245 AM EDT Thu Apr 18 2019
Valid 12Z Sun Apr 21 2019 - 12Z Thu Apr 25 2019
...Overview and Guidance Preferences...
Increasingly zonal flow over the CONUS by early next week will
lead to lower predictability of generally weaker frontal systems
next week. Continued a blended/consensus approach to minimize
future shifts in system timing/track. Ensembles generally agree
that troughing will slowly exit the Southwest/Four Corners region
early next week and push into the southern Plains as the northern
stream along the US/Canadian border wobbles downstream of an upper
low in the Gulf of Alaska. 12Z ECMWF was closest to the ensemble
consensus, which differed on the speed of the western trough
(ECMWF ensembles generally quicker than the GEFS/Canadian
members). A deterministic blend early in the period gave way to a
blend of the ECMWF ensemble mean (which has trended slower by next
Wed/Thu toward the GEFS mean) with the slower 00Z ECMWF and 18Z
GEFS mean. Questions in specifics (e.g., will low pressure lift
toward the Great Lakes next Wed or stay flatter?) will remain
unanswered for now.
...Weather Highlights/Threats...
The overall shortwave trough aloft affecting the western/central
U.S. and associated surface features will spread rain and high
elevation snow over central latitudes of the West and then
rainfall of varying intensity over central/southern parts of the
Plains and possibly the Mississippi Valley. Confidence in
specifics of this precipitation area is lower than desired due to
the guidance spread for details aloft. There is a signal for
low-level upslope flow to provide some precipitation enhancement
along the eastern slopes of the central Rockies around Sun (snow
at relatively high elevations). Thereafter ensemble guidance shows
relatively higher probabilities for locally heavy rainfall over
the southern Plains which should see a multi-day feed of moisture
from the Gulf, though placement differences (especially
north-south) exist between the farther south ECMWF ensembles and
farther north GEFS ensembles. The Pacific Northwest should see
periods of generally light precipitation from Mon onward as
multiple fronts approach/cross the region.
Behind the exiting eastern system a brief period of chilly
temperatures are expected for the Mississippi Valley/East on
Sunday. Thereafter, temperatures should otherwise be near to above
normal given the flatter trend of flow aloft. Exception may be
over Texas later next week given extensive cloud cover and
rainfall. Expect the highest warm anomalies over the West and into
the northern High Plains with one or more days of max and/or min
readings 10-20F above normal.
Fracasso/Rausch
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