Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1149 AM EDT Wed May 09 2018
Valid 12Z Sat May 12 2018 - 12Z Wed May 16 2018
...Pattern overview...
An upper low is expected to meander over the Great Basin this
weekend into early next week with zonal flow across the
central/eastern states. An upper low just north of Hudson Bay will
promote broad cyclonic flow over southeastern Canada and along our
shared border into/through the Great Lakes and Northeast. At the
surface, a front associated with the upper low in the west will
slowly push out of the Rockies by next week as a Pacific front
finally approaches the Washington/Oregon coast around next
Tuesday. The eastern extent of the front in the Rockies will ebb
and flow northward and southward in the east as successive waves
exit off the coast into the Atlantic.
...Guidance/uncertainty assessment/preferences...
Model/ensemble consensus remains quite well-clustered to star the
medium range period, keeping an upper low back across the Great
Basin Sat-Mon and a multi-model deterministic blend (00Z
GFS/ECMWF/UKMET/CMC and 06Z GFS) was used as the starting point.
This blend also resolved the stationary front (and a few waves
along it) well as some expected timing and latitudinal differences
remain. One wave of low pressure will exit the Northeast on
Saturday as another will push through the Ohio valley on Sunday.
The GFS was first to depart from the good clustering well
northwest of the CONUS just south of the Gulf of Alaska and
dropped it from the consensus by after next Monday due to its
differences in system evolution there as well as downstream
through southern Canada where it appeared much too strong with
height falls into the Great Lakes next Tue-Wed. The 00Z
ECMWF/Canadian best represented the ensemble clustering of the 06Z
GEFS and 00Z ECMWF ensemble mean which was used as the starting
point for next Tue/Wed as the upper low in the west weakens and
lifts out due to the kicker system in the Pacific just off the
west coast. The 00Z ECMWF ensemble mean still appeared too quick
with the weakening lead system and approaching Pacific system but
a blend with the slower ECMWF and much slower Canadian offset it.
With an overall trend toward slower system progression, opted to
rely more on the slower solutions again today through the
mid-latitudes.
...Sensible weather/threats highlights...
The wavy/stationary frontal boundary extending from the central
Rockies and adjacent plains to the Great Lakes/Northeast from Sat
onward will serve as a focus for scattered to numerous showers and
thunderstorms over a large area. Models continue to suggest the
potential for areas of heavy rain (and high elevations snows)
across portions of the central Rockies Sat-Sun (especially
northeastern Nevada eastward into Wyoming) as the vigorous upper
trough amplifies to the west and a surface low develops along the
front across the central plains, enhancing low-level easterly
upslope flow. A number of deterministic solutions show the
potential for localized multi-day totals of at least a few inches
(straddling the short range into the medium range). As the front
exits toward the plains next week, precipitation will expand from
northern Texas northward to Kansas and northeastward along the
frontal boundary into the Corn Belt. By next Tue-Wed, convective
activity may once again expand northward to the northern
plains/Upper Midwest as upper energy finally begins to exit the
Rockies, enhancing upper-level support for precip, and the central
plains' surface front lifts northward as a warm front.
Farther east, broad southerly flow across the plains will
transport moisture north and lift it across the frontal boundary.
Models show potential for multiple rounds of convection along the
front Sat-Sun across portions of the Midwest/Great Lakes and into
the Northeast/Mid-Atlantic, with the potential for areas of
locally heavy rainfall. Confidence in the specific locations of
any heavy rain is relatively low at this time, and would likely be
determined by mesoscale/convective processes. Farther south, a
dissipating frontal boundary and developing upper low will focus
increasingly widespread shower and thunderstorm activity across
the southern/central Florida Peninsula from late this week into
early next week. Areas of heavy rainfall will be possible given
the multi-day nature of the event and high Pwat values (+2 to +3
standard deviations).
High temperatures are expected to be 10 to 20 deg F above average
across much of the southern tier on Saturday, with the center of
greatest anomalies shifting eastward to the lower Mississippi
valley, Southeast, and Mid-Atlantic by Sun-Mon (+10 to + 15 deg F
anomalies). Record high temperatures will likely shift eastward
out of the southern Plains Saturday (near 100 degrees along the
Red River between TX/OK) and into the Mississippi valley Sat-Tue
next week. Temperatures to the north near the Canadian border will
be below average on Saturday with a trend toward average or
slightly above average by next Monday as the colder temperatures
remain confined to Canada.
Fracasso/Ryan
Associated WPC medium range forecasts of 500 mb heights, surface
systems, weather grids, quantitative precipitation forecasts, and
winter weather outlook snow/sleet probabilities can be 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