Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 AM EDT Tue May 14 2019
Valid 12Z Fri May 17 2019 - 12Z Tue May 21 2019
...Overview and Guidance evaluation/preferences...
Mean troughing aloft will bring a wet period to a large portion of
the western U.S. and Plains states during the weekend to early
next week. Much of the western US to the northern high Plains
will turn colder, with much of the lower MS Valley and TN Valley
to the southeast turning warm to hot.
Guidance is steadily trending toward resolving the leading strong
trough progressing from the northern Rockies onto the Plains by
day 3 Fri and progressing across the northern Plains thereafter.
By day 4 Sat the operational GFS runs become faster than the other
models in moving the closed low sfc/aloft out of the Plains across
the upper MS Valley and upper Great Lakes. The 12z ECMWF/00z
UKMET/12z Canadian suggest that southern trough energy could form
another upper low that reaches the Plains late Sat-Sun, which
leads to slightly slower forward motion, which is preferred.
There is good agreement that upstream flow will bring another
round of strong height falls to the western states this weekend,
continuing inland early week across the Great Basin to the
northern high Plains. The 12z ECMWF and 12-18z GFS indicate a
deeper low in the eastern Pacific early next week, which has
potential to reach CA Tue 21 May.
Models/ensembles increasingly diverge for northeastern/mid
Atlantic U.S. flow, highlighted by a deep New England/Mid-Atlantic
trough aloft in the 00Z UKMET versus a strong ridge in recent GFS
runs. The 12Z ECMWF/12z Canadian had lower heights than the GFS,
which were given a little more weight. Teleconnections relative to
the negative height anomaly center associated with the West Coast
mean trough favor modest cyclonic flow aloft, thus the GFS was
given less weight. More weighting was given to the 12z ECMWF
mean/18z GEFS Mean to avoid committing to any particular model
solution given continuing spread.
...Weather Highlights/Threats...
Over the West enhanced precipitation is expected with the mid
level closed low over the northern Rockies and High plains
Fri-Sat. The next significant precip event is expected Sat-Sun
extending from favored terrain in northern-central California into
the northern Rockies and then the northern great Basin to the the
high Plains Mon 20 May-Tue 21 May. This period is expected to be
unusually wet for so late in the wet season from CA to the
northern Rockies. The greatest potential for high elevation snow
should be in the northern Sierra Nevada.
Progression of the initial western storm will generate areas of
heavy rain/thunderstorms over the central to southern Plains over
the weekend, with the Storm Prediction Center also indicating
severe weather potential for Fri-Sat.
Widespread rain is expected over the northern Plains to upper MS
Valley this weekend.
Well below normal highs (clusters of minus 10-20F anomalies) over
California/ Nevada/Utah on day 3 Fri will spread northeast over
the weekend into the northern Plains and remain in place through
early next week. Clouds and precip should keep morning lows mostly
a few degrees below normal with only isolated locations ten or
more degrees below normal this weekend into early next week.
Well above normal highs will progress from the Plains Fri and
gradually relocate into the MS Valley and Oh Valley over the
weekend and the southern mid Atlantic into the southeast early
next week, with temps commonly 10-15 degrees above normal in the
core of the warmest air. From Sat-Mon temperatures over the mid
Atlantic states are very uncertain given a wide range of
possibilities for frontal position within a region of strong
temperature contrast.
Petersen
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