Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1202 PM EDT Mon Aug 12 2019
Valid 12Z Thu Aug 15 2019 - 12Z Mon Aug 19 2019
...Overview and Guidance/Predictability Assessment...
Upper pattern over the Lower 48 will be guided by strong ridging
over southwestern Alaska and over the Desert Southwest, which
favors troughing over western Canada into the Pacific Northwest.
In the east, weak/modest troughing to start the period will
transition to weak ridging in response to the digging western
trough. For several cycles, the models/ensembles have struggled to
agree on how the high latitude flow from Alaska and far
northwestern Canada will evolve with quite disparate solutions
seen. The cacophony of opinions in the 00Z guidance mostly was
divided between the ECMWF and its ensemble members vs most
everything else (GFS/Canadian and their ensembles) with respect to
timing/track of the flow out of Alaska. However, trends in the
preceding days favors something along the line of the 00Z
ECMWF/GFS with a nearly closed low moving through Oregon Friday
(06Z GFS and 00Z UKMET appeared much too weak and the 00Z Canadian
seemed a bit too quick). From that point onward, only the ECMWF
appeared to be the most reasonable solution as the 00Z GFS
appeared too quick with the flow out of Alaska (though the ECMWF
may be too slow). By next Sun/Mon, opted to rely most on the 00Z
ECMWF ensemble mean that, despite its larger-than-typical shifts
over the previous few runs, offered a solution that fit the
favored pattern of renewed height falls into British Columbia that
was at least supported by the Canadian ensembles as well.
Confidence was lower than normal in the Pacific Northwest but
otherwise near to above average over the central and eastern
states, respectively.
...Weather Highlights/Threats...
A warm front over the central/high plains may act as a focus out
of the short range for locally heavy rainfall as a shortwave drops
southeastward Thursday. This front will slowly lift northeastward
and eventually dissipate as a baroclinic zone remains over much of
the Northwest, enhanced as the shortwave or weak upper low move
through on Friday. This will increase shower activity over WA/OR
into ID/MT with over an inch possible in favored areas. Elsewhere,
a lingering stationary boundary in the Southeast will provide
broad-scale lift in a juicy environment for mostly afternoon-based
showers/storms that may yield some locally heavy totals. Models
may be suffering from an overzealous output of precipitation but
overall setup will nonetheless be favorable for at least some rain
for most areas.
Temperatures in the Southwest and southern Plains will be hot and
near record highs (and warm lows with many areas only in the upper
70s/80s) Thu/Fri. This may expand into the Ohio Valley/Great Lakes
region next weekend as heights rise in the east. Heat indices will
again rise in the low 100s for many areas in the South and over
110 in the Southwest as monsoonal rainfall will be virtually
non-existent this weekend into Monday.
Fracasso
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