Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Fri May 18 2018
Valid 12Z Mon May 21 2018 - 12Z Fri May 25 2018
...Guidance and Predictability Assessment...
Overall, especially in the first half of the medium range period,
there continues to be fair agreement in models on the overall
pattern. In the beginning of the week, a robust upper trough will
move over the West Coast on Monday and become a closed off low as
it meanders toward the Desert Southwest by Tuesday. The 00Z
ECMWF/CMC/06Z GFS are in agreement with the evolution of this
trough; however, the GFS both the 06Z and 00Z runs have showed the
primary closed low breaking off along with another piece of energy
sliding southward from the trough. With the 00Z ECMWF/CMC in
agreement and the GFS and UKMET showing these two closed lows,
decided to blend all four solutions with slightly more weight
given to the ECMWF and CMC solutions.
By Day 5/Wednesday, models are still in agreement with the overall
pattern with an open wave trough over the western U.S. and the
upper ridge beginning to weaken across western Canada and the
central U.S. Much like what was occurring on day 4/Tuesday, there
are difference in the details especially between the 00Z ECMWF/CMC
and 06Z/00Z GFS. Both the 00Z GEFS and 00Z ECENS are trending
toward the 00Z ECMWF/CMC in terms of the orientation of the
trough. Consequently, by day 5, dropped the operational GFS and
began to increase the usage of the 00Z GEFS/ECENS while keeping
the 00Z ECMWF/CMC.
By Day 6 and 7/Thursday and Friday, both the 00Z ECENS/GEFS and
00Z ECMWF/CMC show another trough approaching the West Coast. The
06Z GFS and 00Z GFS have both been inconsistent with itself and
other models with handling this trough. The 00Z ECMWF is much
faster with with the trough--whereas the 12Z ECMWF matches fairly
well with the 00Z CMC. Farther east, the ECENS/GEFS were
comparable in handling the northern stream cyclonic flow and the
effects on surface fronts/waves. For the tropical wave, the 00Z
CMC was faster and more aggressive with it--as was the 00Z GFS.
The 06Z GFS has trended slower and having a more broad wave as the
ECMWF has consistently suggested. Thus the blend was based on 00Z
(12Z on day 7)ECMWF/00Z CMC along with the 00Z ECENS/GEFS.
...Weather/Threats Highlights...
With the first upper trough moving across the West Coast,
scattered precipitation will occur across the Great Basin and
Intermountain West. An increase in precipitation can be expected
especially in the higher elevations of north and central
California and through the northern and central Rockies. In
addition, convection could occur along the southern Plains on
Monday and track eastward along the warm front lifting into the
central Plains/MS Valley. As the next trough approaches the
Pacific Northwest, an increase in precipitation can be expected
especially in eastern OR/WA. Confidence still remains low with the
tropical feature in the Caribbean and how it will impact
Florida--but an increase in moisture from this feature could occur
and thus would increase precipitation coverage.
In terms of temperatures, most of the country will slightly warmer
than normal. There is a potential to break high minimum
temperature records across the Tennessee Valley and into the
Mid-Atlantic in the beginning of the week. Florida and the
immediate coastline along California will remain below average in
terms of high temperatures.
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be found at:
http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
Reinhart/Rausch