Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Mon May 13 2019
Valid 12Z Thu May 16 2019 - 12Z Mon May 20 2019
...Overview...
Mean troughing aloft will bring very active weather to a large
portion of the western and central U.S. during the Thu-Mon period.
One or more periods of heavy precipitation (including some high
elevation snow) will be possible from the West Coast states
through the northern 3/4 of the Interior West/Rockies, with
episodes of heavy rainfall and severe weather possible over the
central U.S. Meanwhile guidance spread yields very low confidence
in specifics over the East, aside from likely existence of a
strong front that will separate chilly air to the north from very
warm air farther south.
...Guidance evaluation/preferences...
Guidance is steadily trending toward resolving the leading strong
trough reaching the West Coast by day 3 Thu and progressing inland
thereafter. ECMWF/ECMWF mean runs were earlier to catch onto the
separated nature of the trough with embedded upper low during
Thu-Fri. By day 5 Sat operational GFS runs become questionable
with their speed of height falls crossing the northern half of the
Plains. The 06Z FV3 GFS compares better to remaining consensus.
Some solutions including the ECMWF/UKMET/FV3 suggest that southern
trough energy (whether from the initial upper low or otherwise)
could form another upper low that reaches the Plains late Sat-Sun.
In relative terms there is good agreement that upstream flow will
bring another round of strong height falls to the West Coast by
days 6-7 Sun-Mon. Interestingly, here the ECMWF/ECMWF mean have
shown a bit of a trend toward the GFS/GEFS mean that have been
deeper over the past day or so.
Models/ensembles increasingly diverge for eastern U.S. flow
details, highlighted by a deep New England/Mid-Atlantic trough
aloft in the 00Z ECMWF versus a strong ridge in GFS runs. The 00Z
ECMWF is fairly extreme relative to its ensembles but not an
outlier, while the previous ECMWF run was less extreme and closer
to the current ECMWF mean. Remaining solutions show flat to
slightly cyclonic flow. Teleconnections relative to the negative
height anomaly center associated with the West Coast mean trough
favor modest cyclonic flow aloft--but one cannot rule out a
transient ridge that could still yield slight troughing over a
multi-day time frame. Thus preferences lean only slightly toward
the ECMWF mean scenario relative to GEFS mean late in the period.
Based on forecast considerations the updated forecast blend
incorporated an operational model blend (plus the FV3 GFS) early
in the period and then rapidly increased 00Z ECMWF mean/06Z GEFS
mean input with time.
...Weather Highlights/Threats...
Over the West expect two primary episodes of enhanced
precipitation, one with a system reaching near the coast at the
start of the period Thu and then another affecting the West Coast
and points inland Sat onward. Greatest 5-day totals should extend
from favored terrain in northern-central California into the
northern Rockies--and at some locations could be quite high for so
late in the wet season. Best potential for high elevation snow
should be early and late period in the Sierra Nevada and Fri or
Fri night onward in the Rockies.
Progression of the initial western storm will generate areas of
heavy rain/thunderstorms over the central U.S. by the weekend,
with the Storm Prediction Center also indicating potential for
severe weather for at least a couple days. Ahead of this
activity, a wavy front draped over the Midwest could serve as a
focus for locally heavy rainfall late this week.
Over the East, a lingering low latitude front may bring periods of
rain to the southern Florida Peninsula. Farther north one or more
fronts may bring some rainfall but confidence is low in specifics.
As the West Coast mean trough aloft becomes established, well
below normal highs over California/western Nevada on day 3 Thu
(minus 10-20F anomalies) will spread northeast with time and by
Sat-Mon cover an area from the West Coast into the northern
Plains. Clouds and precip will keep morning lows closer to normal
over this area. Leading warmth will progress from the
Rockies/Plains and likely moderate gradually as it reaches the
East. Some locations over the north-central Plains may see highs
10-20F above normal Thu-Fri. Temperature distribution over parts
of the East is very uncertain given a wide range of possible
frontal location near the East Coast. Ensemble spread reaches at
least 30F for highs at some locations, depending on whether the
front is to the north or south.
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