Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1150 AM EDT Sat Jul 06 2019
Valid 12Z Tue Jul 09 2019 - 12Z Sat Jul 13 2019
...16Z Update...
Overall, no major changes were needed to continuity for this
forecast cycle, as the large scale pattern appears well handled by
model guidance but with some variability in smaller scales. A
multi-model blend was utilized for the medium range forecasts with
this reasonably good model agreement. Did eventually eliminate the
00Z CMC from the blend due to its handling of the shortwave in the
Great Lakes region on day 5/Thu--the trough was held west a bit,
allowing it to amplify on Fri along the Eastern Seaboard unlike
other guidance.
Models continue to indicate the likelihood of a surface low
meandering near the Central Gulf Coast through the medium range
period even after a surface front dies in its vicinity by midweek.
The intensity of the surface low remains in question, with the 00Z
ECMWF much stronger than any other guidance except for a handful
of its own ensemble members (00Z and 06Z GEFS members remain
weaker, as do the deterministic GFS runs). Yesterday's 12Z ECMWF
also had a weaker system. With this run-to-run variability, low
confidence remains regarding the intensity of the surface low.
For more details, please see the previous discussion below.
Tate
...Previous Discussion...
...Overview and Guidance/Predictability Assessment...
The latest guidance continues to provide reasonably high
confidence in the large scale pattern evolution but there is
embedded uncertainty in the medium to smaller scale details.
Consensus shows southern tier ridging aloft will shift its
emphasis to the southern Rockies/Four Corners region by mid-late
week, likely reaching its greatest strength around Thu. As this
occurs a shortwave crossing the northern tier will head into the
eastern states. Meanwhile most guidance is still suggesting that
a current Mississippi Valley impulse aloft may track to the
Southeast/Gulf Coast and interact with a wavy front to yield a
surface low near the Gulf Coast. An upper low well offshore the
Pacific Northwest should track west of Vancouver Island as it
weakens and sends some of its energy eastward. Upstream flow will
likely feed into a lingering mean trough off the West Coast into
the weekend.
Comparison among the 12-18Z model/ensemble guidance reveals that
the GFS may still be having some fast timing issues in the
northern stream--with eventual ejection of the upper low tracking
west of Vancouver Island as well as for the shortwave reaching the
eastern states. The 18Z GFS was not as extreme as the 12Z run for
the eastern shortwave and now the new 00Z GFS has furthered the
trend toward consensus. The 00Z GFS still looks fast to eject
eastern Pacific upper low energy after early Wed though. By the
latter half of the period, the various uncertainties with
individual shortwaves from the North Pacific through southern
Canada/northern U.S. will limit confidence in surface details
across the northern Plains and vicinity.
Albeit with typical run-to-run variability, the GFS/ECMWF and
their ensemble means (plus the UKMET through the end of its recent
runs) have been fairly consistent in showing the potential for a
surface low most likely near the Florida Panhandle as an upper
level impulse interacts with a wavy front over the region. A
consensus blend yields a solution fairly close to continuity. The
small scale of this evolution keeps confidence/predictability in
specifics fairly low. The approach of a front from the north by
Fri-Sat could provide added uncertainty in the forecast late in
the period.
The manual forecast reflects an operational model blend early and
then a model/mean blend that trends to majority weight of the 18Z
GEFS/12Z ECMWF means by day 7 Sat. GFS input is 25 percent or
lower due to the aforementioned questionable timing of northern
stream features after early Wed.
...Weather Highlights/Threats...
There is a continued focus for heavy rain potential within an area
from the southeastern coast through Florida and possibly to the
central Gulf Coast. A wavy front will be positioned over the area
by the start of the period early Tue, and while the front should
weaken with time it appears likely that a lingering surface low
may track in the vicinity of the Florida Panhandle. Exact details
of the surface evolution and rainfall emphasis are quite uncertain
given the small scale of the feature and typical difficulty that
guidance can have in modeling rainfall associated with such
systems. Meanwhile a front progressing from the northern Plains
into the eastern/southern U.S. should produce periods of organized
showers and thunderstorms. Expect best potential for heaviest
rainfall to be over/near the Upper Mississippi Valley/Midwest and
then portions of the Northeast. Toward Fri-Sat the southern part
of the front could begin to interact with the moisture over the
Southeast. The upper low offshore the Pacific Northwest may track
just far enough east to bring some moisture to the region
primarily around Wed-Thu. Expect mostly light to isolated
moderate rainfall from coastal Washington/Oregon into the
Washington Cascades.
Behind the initial northern Plains front, some northern Plains
locations should see highs 5-15F below normal Tue into Wed. After
midweek the pattern will favor near to above normal temperatures
over a majority of the West and by Fri-Sat extending into the
northern Plains. Anomalies will likely be higher for morning lows
versus afternoon highs. There will also be a band of warmth
(mostly single-digit anomalies) ahead of the advancing
central-eastern U.S. front. Locations from the southern Plains
through Lower Mississippi Valley may see heat index values high
enough to be of concern through midweek.
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