Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Fri Jul 05 2019
Valid 12Z Mon Jul 08 2019 - 12Z Fri Jul 12 2019
...16Z Update...
Overnight model guidance shows a similar overall pattern as in the
previous forecast. Regarding the shortwave moving through the
northern tier, it seems well-handled on Mon/Tue, but the GFS 00Z
and 06Z deterministic runs have the shortwave a little flatter and
faster by Wed than other guidance including the GEFS mean, so
leaned away from those solutions. Like previous runs, the GFS
deterministic runs were on the eastern/faster side of the guidance
envelope with the trough in the eastern Pacific and coming into
the Pacific Northwest, so went more toward the ECMWF/EC mean with
incorporation of the GEFS mean, much like the previous forecast.
Another interesting feature to watch is the possibility of a low
along or near the Gulf Coast for the second half of the week. It
appears energy from an MCV currently in Missouri will propagate
southeastward early in the week and possibly join up with a
surface front that will gradually die along the central Gulf Coast
and the Georgia/South Carolina Atlantic coast. The 00Z ECMWF run
is most bullish with the low development, and confidence in the
strength of the feature is low. But it does appear there could be
a low in the extreme Southeast U.S. or in northern portions of the
Gulf of Mexico on Thu/Fri with a fair amount of ensemble support.
For more information and sensible weather highlights, please see
the previous discussion.
Tate
...Previous Discussion...
...Overview and Guidance/Predictability Assessment...
Most guidance shows rapid ejection of an upper trough initially
just inland from the West Coast. The remaining shortwave should
traverse the northern tier states and eventually reach the eastern
U.S. while southern tier ridging aloft shifts its emphasis from
the southern Plains on Mon to the southern Rockies/Four Corners
region by mid-late week. A northern Pacific upper low may
approach the Pacific Northwest but with some uncertainty over its
ultimate path. One other evolution to be monitored will be
diffuse Plains energy that models eventually bring into the
extreme Southeast U.S. and eastern Gulf of Mexico and which may
interact with waviness along a weakening surface front.
Based on the array of guidance available through the 12-18Z
cycles, the primary forecast consideration was to underweight the
operational GFS runs which became faster/eastward versus consensus
(including recent GEFS means) for the upper low nearing the
Pacific Northwest. The 12Z GFS was slightly slower than the 18Z
run. Upstream details over and north of the North Pacific remain
uncertain but at least the new 00Z GFS has trended favorably
slower with the upper low. Meanwhile the 12Z ECMWF seemed a bit
slow/southwest with the upper low and the new CMC/UKMET/GEFS mean
all reinforce that idea.
Over the past day the model consensus has become better defined
with the ejecting western U.S. shortwave as it reaches the
northern Plains/Upper Mississippi Valley by midweek and then
settles into the East as the upper ridge strengthens over the
southern Rockies. In spite of the favorable 00Z GFS adjustment
off the Pacific Northwest it is still on the fast side of the
envelope with the northern tier shortwave by Wed.
The small scale of features involved lends itself to below average
predictability for what may evolve at the surface and aloft over
the extreme Southeast and eastern Gulf of Mexico toward mid-late
week. The GFS/ECMWF and their means offer fairly similar ideas
but other solutions provide alternative scenarios.
In light of above considerations, the updated forecast started
with a model consensus early in the period--though splitting a
portion of the 12Z GFS weight with the 18Z GEFS mean. Then the
blend increasingly emphasized the 18Z GEFS/12Z ECMWF means with
continued minority input of the 12Z ECMWF.
...Weather Highlights/Threats...
A front making steady progress from the northern Plains early in
the week to the eastern states by late week will provide a focus
for organized showers and thunderstorms of varying intensity
through the period. Best potential for heaviest rainfall should
be over parts of the northern half of the Plains and Upper
Mississippi Valley with a secondary maximum over the Northeast.
Some of the northern Plains activity early in the week may be
severe. Check the latest SPC outlooks for additional severe info.
Another area with heavy rain potential will be from the Southeast
coast through parts of Florida and along the eastern/central Gulf
Coast. During Mon-Wed there is reasonable consensus toward a wavy
front settling over this region. However by late week there is
more uncertainty over what may become of potential lingering
waviness as the front weakens. The upper low offshore the Pacific
Northwest may reach close enough to the region to bring in some
moisture, with mostly light to locally moderate rainfall.
The western U.S. will likely see a warming trend after starting
out below normal Mon-Tue as the initial trough aloft crosses the
region. Some of the cool daytime highs will extend into the
northern Plains. Best focus for above normal readings later in
the week should be over the interior West as the upper pattern
evolves toward strong ridging over the southern Rockies/Four
Corners and troughing off the Northwest. Farther eastward
temperatures will vary in relation to the progress of the northern
Plains to eastern U.S. front. Areas from the southern Plains to
interior Southeast will see highs up to a few degrees F above
normal one or more days. Such anomalies are not exceptional but
it will still be hot enough with sufficient humidity to yield
fairly high heat index values.
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