SHORT TERM [Thursday through Friday Night]...The trajectory of
the CENTER of the surface high pressure looks to be from near
Missouri around 12z Thursday to offshore of North Carolina by 18z
Friday. This will leave our forecast area with a more peripheral
shot of cooler, drier AIR in the Short Term period. 850mb TEMPS
for Thursday and Thursday Night look to be about +4C to +7C based
on SREF and GFS ENSEMBLE means. This should result in highs in the
60s and lows in the upper 30s. For Friday, there is some more
uncertainty related to UPSTREAM phasing (or lack thereof) of
several MID-upper level shortwave troughs. At issue is the
interaction of a subtle shortwave in the process of amplifying
over Idaho at the moment, and an upper-level closed low near Baja
California. The 08.12z ECMWF takes the northern stream wave and
shifts it quickly ESE towards our area on the southern end of a
broadly amplifying TROUGH in the eastern US, with the Baja closed
low spinning well away around 20N/110W. Meanwhile, the 08.12z GFS
phases the two waves in the Desert Southwest or northern Mexico as
an elongated trough. The recent runs of the UKMET and GEM look
similar to the ECMWF, and that is the direction the forecast
trends in. With the closed low near Baja California drifting south
on all the models, it does not seem LIKELY that there will be much
interaction with a very progressive, low-AMPLITUDE wave far to the
north. The end result is that there is likely to be a bit more QG
CONVERGENCE and broader mechanisms for ascent, which translates
into a cloudier forecast for Friday and Friday Night with some low
RAIN chances. The ECMWF QG convergence in the 700-300mb layer is
actually quite strong around 06z Saturday over our forecast area,
but there are some questions as to how fast a fairly deep layer of
mid-level dry air can be eroded. Therefore, the previous forecast
of slight CHANCE POPS was maintained for now.