Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1256 AM EDT Fri Jun 29 2018
Valid 12Z Mon Jul 02 2018 - 12Z Fri Jul 06 2018
...Long duration heat wave to continue over the northeastern
portion of the lower 48...
...Heavy rain threat for the northern Gulf Coast...
...Overview...
Models and ensembles remain in good agreement next week with the
upper pattern that will feature an expanding upper high that will
likely stretch from southern California northeastward to the
Appalachians. This will support well above average temperatures
over the eastern Great Lakes/Northeast/Mid-Atlantic with daily
record highs likely to be broken. One upper low will linger over
the Pacific Northwest and another off the southeast coast with yet
another to the southwest of the upper high over the Gulf Coast. A
surface frontal boundary will lie astride the north/northwest side
of the expanding 594-597dm (maybe 600dm?) upper high -- roughly
from the Pacific/Interior West through the northern Plains into
the Great Lakes.
...Guidance evaluation/preferences...
The latest deterministic models formed a good starting point for
the forecast (GFS/ECMWF/Canadian/UKMET) though the 12Z/18Z GFS and
most GEFS members were slower than the rest of the
models/ensembles to bring the upper low southward toward western
Washington early next week. After that, GFS/ECMWF fall within the
ensemble spread and a blend with their ensembles tempered the
differences out to next Friday. Biggest trouble spot may be off
the east coast with the upper low -- both in its track and
strength (and eventual impact via precipitation).
...Weather threats/highlights...
Ascendant upper high will force precipitation around its
circulation, firstly in the Northwest in conjunction with
southwest flow ahead of the upper low atop the surface boundary.
This will extend eastward across Montana into the Plains and Great
Lakes as the surface low wanders through southern Canada. A lead
front into the Northeast next Mon/Tue will do little but knock
temperatures a few degrees and perhaps touch off a shower/storm as
it dissipates by Wednesday. The Southeast into Florida may see an
increase in precipitation should the Atlantic upper low wobble
westward toward the Carolinas later in the week. Finally, the most
significant precipitation may lie along the Gulf Coast from LA/MS
westward to the upper Texas coast as a weakness drifts westward.
ECMWF/UKMET/Canadian seemed a bit aggressive/wet but with plenty
of moisture to tap (PW anomalies about +2 to +3 sigma) heavy
rainfall is certainly a possibility.
Temperatures will be near/above records in the Northeast early in
the week with a slow downward trend (but still above average) by
late in the week as the core of the upper high moves
southwestward. Widespread 90s are forecast for most of the lower
48 states while cooler than average temperatures are expected for
the Northwest and also along the Gulf Coast (especially max temps)
given extensive cloud cover and rain.
Fracasso
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities are 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