Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
403 am MDT Thursday Jul 30 2015
Synopsis...a ridge of high pressure will remain over the region
through the end of the week...then shift eastward early next week.
Moisture will slowly increase across southern Utah today and
Friday...then spread slowly northward late in the weekend.
Short term (through 00z sunday)...the ridge axis remains in place across
central Utah stretching from the Southern Plains to northern
California. To the north of this axis the air mass remains very dry
while to the south there has been a little increase in middle to high
level moisture. Despite precipitable waters coming up across extreme southern Utah
today and Friday...forecast soundings show that this increase is due
primarily to moisture aloft above 400mb. There is a weak disturbance
that moves northward from Arizona towards the Utah border by late
today and into extreme southern Utah near Lake Powell by Friday so
kept some meager probability of precipitation there. Otherwise...low levels remain rather
dry as northerly flow remains in place below about 600mb
..therefore have lowered probability of precipitation from about Garfield County north and
west. What little convective activity that does form today and for
that matter Friday will likely be of the high-based variety
thunderstorms with little if any rain but instead gusty winds.
Temperatures across the northern portions of the County Warning Area should warm
another several degrees today as the 850mb temperatures warm 2-3 degrees
celsius and the 700mb temperatures warm about 2 degrees celsius. Across the
south the temperatures will be quite similar to yesterday with most
places within a degree or two. Some places may be a little cooler
especially near the Arizona border where extra cloud cover could
inhibit strong warming.
Saturday the moisture inches a little farther north but still not
impressed as northerly winds will remain below 600mb across most of
the County Warning Area. Temperatures at 700mb peak across northern Utah but will
remain stable across the south.
Long term (after 00z sunday)...GFS and European model (ecmwf) in good agreement
early in period...with upper ridge axis centered over forecast
area...but then model solutions diverge. GFS shifts ridge axis
eastward...allowing middle level flow to turn southwesterly...
facilitating a northward push of moisture through Monday...though
lower levels of atmosphere remain rather dry. European model (ecmwf) elongates
ridge to south of forecast area early next week...promoting a
more westerly...drier flow. European model (ecmwf) does seem to be erratic...locking
on to convective complexes and deepening them with time. Given
little spread in GFS ensemble solutions...that are generally in
line with deterministic GFS...will go with GFS solution.
Thus...will keep existing slight chance wording for much of
forecast area Sunday and Monday. With precipitable water values
remaining at or below three quarters of an inch and relatively dry
lower levels...do not see much of a flash flood threat during
With GFS indicating some drier air working into northern and
western portions of forecast area Tuesday and Wednesday...will
indicate less coverage over northwest half of Utah.
Aviation...light southeast winds at slc terminal will shift to
northwest between 15 and 16z this morning with a 40 percent chance
of holding off until 17z. No other weather concerns through the taf
Fire weather...the main fire weather concern over the next few days
will be the potential for dry lightning. Forecast soundings show
that the increasing moisture will be upstairs and that the lower
levels remain rather dry. There is no clear cut organized system
other than a weak disturbance that skirts the southern zones
Friday afternoon to get an area of organized convection...so for
the time being expect dry thunderstorms to be generally isolated
through the weekend. The most widespread activity...across the
entire County Warning Area...is expected to be Sunday and Monday with a drying
trend setting in Tuesday. Along with drying...winds will increase
over western valleys Tuesday and then become stronger and more
widespread Wednesday and Thursday. Depending on the fuels status
by then any hold over lightning strike starts may have an
opportunity to spread.
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http://weather.Gov/saltlakecity/general/afd_faqs.Php (all lower case)