Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Sun Jul 07 2019
Valid 12Z Wed Jul 10 2019 - 12Z Sun Jul 14 2019
...16Z Update...
While the large-scale pattern remains well-handled, the more
uncertain parts of the forecast continue to be shortwave impulses
moving along the northern stream and the possibility of a low with
potential tropical characteristics remaining in the northeastern
Gulf of Mexico after a front in the vicinity dies in the short
range. Regarding the northern stream, noted that by day 5/Fri the
upper low in north central Canada in the 00Z GFS was oriented
differently than other guidance, which affected shortwave
placement over the weekend, so leaned away from that solution. The
00Z GFS also was stronger with an upper low in the eastern Pacific
by the weekend compared to other models. With the Gulf low, the
ECMWF has been strongest for the past few days. This remains the
case, but GFS runs as well as the UKMET and CMC have consistently
been developing a low now, albeit weaker. There is plenty of
spread in the position of the low as well especially late in the
week, and the 00Z ECMWF is one of the slower and farthest west
solutions. Overall leaned toward the GEFS and EC means by the
weekend so as not to lock in on any one solution, similar to the
previous forecast.
Tate
...Previous Discussion...
...Overview and Guidance/Predictability Assessment...
Guidance continues to show a fairly typical summertime pattern, as
reflected by multi-day means that indicate rather modest mid-level
height anomalies over the lower 48. Consensus and continuity for
the large scale flow are still good while some significant
uncertainties remain for embedded features. Expect upper ridging
to be centered over the southern Rockies/Four Corners and into
parts of the central/southern Plains for most of the period. Mean
troughing should persist over the eastern Pacific while
progressive/low amplitude flow will prevail over the northern tier
states and southern Canada. A fairly sharp shortwave trough will
progress across the northeast quadrant of the lower 48 during the
latter half of the week, followed by flat or broadly cyclonic
flow. Guidance is continuing to suggest potential for a
combination of energy aloft and a surface wave (initially along a
weakening southeastern U.S. front) to yield an area of low
pressure over or near the northeastern Gulf of Mexico.
Looking at the forecast over the eastern Pacific and western U.S.,
recent runs have settled on an agreeable solution for the
weakening upper low west of Vancouver Island as of early Wed.
Model differences increase as one or more upstream shortwaves move
into the eastern Pacific mean trough, with meaningful spread for
the degree of height falls that may occur over the Northwest. To
varying degrees and at different time frames recent GFS/CMC/UKMET
runs suggest stronger shortwave energy could approach or reach the
region than a decent majority of GEFS/ECMWF/CMC ensemble members.
For the time being prefer to lean more to the ensemble means given
the long-term mean ridging forecast over western North America,
but confidence is not especially great.
There are still multiple uncertainties regarding evolution of the
system over/near the northeastern Gulf of Mexico. For example
there are some differences in relative emphasis of mid-level
energy--the impulse originating from the Mississippi Valley versus
another feature farther east. Then after decent model clustering
during Wed-Thu there is rapid east-west divergence for surface low
track. For good measure the 00Z UKMET/CMC both track a separate
wave northeast from the southeastern coast as well. The manual
forecast stays close to continuity that was coordinated yesterday,
with a surface low track over the northeastern Gulf through day 6
Sat and a northward day 7 extrapolation based on the motion of
recent GFS/ECMWF runs.
Elsewhere, clustering is now sufficiently good for a consensus
blend to work well for the shortwave progressing eastward from the
Upper Mississippi Valley. Uncertain shortwave details within
progressive northern U.S./southern Canada flow will continue to
provide difficulty in resolving surface features across the
northern Plains and vicinity.
The updated forecast started with an operational model blend early
and then steadily increased 18Z GEFS/12Z ECMWF mean input so that
day 7 Sun consisted almost entirely of the means, reflecting
late-period preferences along the West Coast and to prevent any
one solution from having too much effect on the blend for the
possible Gulf feature.
...Weather Highlights/Threats...
There is still a heavy rainfall threat over areas from the central
Gulf Coast through Florida and the Southeast but with lower than
average confidence in determining where heaviest rain will occur.
The current most likely track of the possible northeastern Gulf
feature would bring highest totals into the east-central Gulf
region. A weakening front near the Southeast coast around midweek
and then another front settling into the South Fri into the
weekend will have the potential to provide added focus for
enhanced rainfall. Before/as the front settles into the South, it
will produce areas of showers/thunderstorms over the eastern half
of the country with highest totals likely over the Upper Great
Lakes and Northeast which will see the strongest upper dynamics.
One or more waves/fronts over the northern tier may help to
produce some areas of showers and thunderstorms over the northern
Plains/Upper Midwest late week into the weekend. Areas from the
Pacific Northwest coast into the Washington Cascades will see a
period of light to locally moderate rainfall early in the period.
By Fri-Sun there may be additional rainfall depending on how
shortwaves may eject from the eastern Pacific mean trough aloft.
Behind the front progressing from the Midwest/Plains into the
East/South, the northern-central Plains should see highs 5-10F
below normal on Wed. Otherwise a large portion of the lower 48
should see near to above normal temperatures for most of the
period. In general anomalies should be greater for morning lows
versus daytime highs. The Intermountain West/northern Rockies
should see the most consistent warmth for the five-day period.
Fairly high heat index values over parts of the southern Plains
through Lower Mississippi Valley should trend lower after Wed.
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