Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EDT Mon May 13 2019
Valid 12Z Thu May 16 2019 - 12Z Mon May 20 2019
Tonight's forecasts were derived from a blend of the latest 12z
ECMWF/18z GEFS Ensemble Means with less weight on the operational
ECMWF and 12z Canadian global days 3-7. The GFS has made
significant run to run changes 12-18z with the Midwest cyclone and
also the east pacific cyclone, a pattern which has continued for
the 00z run. this makes it difficult to have confidence in any
one run of the GFS.
Models have typical timing/amplitude differences with northern
stream energy crossing the central Appalachians/mid
Atlantic/coastal waters Thu, with potential low pressure
development off the coast Fri 17 May. There remains latitudinal
spread as to where low pressure develops on the surface front,
along with typical timing differences, as the ensemble means are a
few hours slower with the frontal progression across Maine. A
blend of the 18z GEFS/12z ECMWF Ensemble means with less weight on
the 12z Canadian/ECMWF models was used until better agreement
develops.
Agreement amongst the models/ensembles supports an amplified upper
trough reaching the west coast Thu 16 May, the southern stream
portion of which then moves inland into the Great Basin Fri 17 May
and then ejects on to the high Plains Sat 18 May. Models differ on
the exact timing and phasing of the trough. Big differences
exist on the intensity of the possible embedded closed low Sat 18
May, as the GFS/ECMWF have shown considerable run to run changes.
As a a result, greater weighting in the forecasts were given to
the ensemble means, including the 12z NAEFS Mean.
Height falls crossing the Rockies on to the Plains by late next
week should induce surface cyclogenesis across the northern Plains
Fri night-Sat, drifting slowly east northeast towards the upper MS
Valley Sun 19 May. Good clustering among the 12z ECMWF/18z GEFS
Mean/12z ECMWF Ensemble Mean/12z Canadian global led to a
consensus of these forecasts. The 00z GFS trended faster and moved
the low well up into Canada by 12z Mon 20 May, making it appear to
be a low probability solution that differs form the 12-18z runs,
other models, and its ensemble mean.
Another low is forecast to develop on the high Plains as the next
wave within the upper trough crosses the Great Basin to the CO/NM
Rockies Mon 20 May.
...Weather Highlights/Threats...
A surface wave and cold front should produce showers over the
northeast Thu before moving off the coast on Fri. Showers/storms
also develop in the vicinity of a stationary front lingering over
the southern Fl peninsula in the middle to latter portions of next
week.
The upper trough should bring increased clouds/precipitation to
the Pacific Northwest and CA, continuing inland into the Great
Basin and northern Rockies Thu 16 May, and then the Plains on Fri
17 May and Sat 18 May. Higher elevation snows develop as colder
air aloft moves into the northwest US, starting in northern
CA/western OR Thu and then spreading into the higher elevations of
the Northern Rockies and northern Great Basin next weekend.
With low pressure moving slowly east across the Plains next
weekend, return moist southerly flow provides fuel to enhance
shower/thunderstorm activity, with potential for locally heavy
rain to develop over portions of central and eastern Texas.
Temperature-wise, highs/lows of 10-15 F above normal are forecast
over the northern to central Plains Thu, progressing into the mid
MS Valley Fri, Ohio Valley Sat/Sun, and central Appalachians next
Mon 20 May.
With the approaching trough, CA/OR/WA will see a reversal to
cooler than normal temperatures in the middle to latter portions
of the week.
Temperatures several degrees above normal develop over the
southeast late in the week and continue through the weekend and
next Mon 20 May.
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