FXUS65 KTWC 261036

Area Forecast Discussion
National Weather Service Tucson AZ
336 AM MST Wed Sep 26 2018

.SYNOPSIS...Mostly dry conditions this week, with a couple of storms
possible near the mountains from time to time. Temperatures remain
above average for late September. Showers and thunderstorms will
return early next week as moisture increases across the area from
the south.


.DISCUSSION...As a trough pushes downstream through the Plains,
we'll gain a northwesterly flow by this afternoon with some
additional drying by later today or tonight. We might hold off
long enough for a few buildups in the eastern mountains, but by
Thursday not much.

High pressure building in from the south and west will push
temperatures to around 7 or 8 degrees above average Thursday and
Friday. Again, one or two more 100 degree days are possible for

Over the weekend our flow will back around to southwesterly and
southerly with a trough near the west coast and weak high pressure
overhead and east. This will open the area up to any moisture
increase we might manage from the southwest and south, and as it
so happens we are lining something up. Even as our precipitable
water threatens to drop well below 1 inch over the next 24 hours,
much deeper moisture remains over Sinaloa and southern Sonora. We
also have newly minted Tropical Storm Rosa (currently well south
of Baja) that is expected to increase to hurricane strength and
drift westward before turning northward over the weekend. This
will likely help push some of that deeper moisture in our general
direction Sunday and Monday.

It's a complex pattern that it's running into. A rex block cycling
into a deep west coast trough early next week with all sorts of
timing issues that will govern the final track of the remnants of
the storm. Right now, we still play the percentages. That means
with an increase in scenarios that bring an upswing in shower and
thunderstorm activity our pops are increasing guided by a blend of
operational and ensemble means, with a dose of ECMWF for good
measure. NAEFS mean precipitable water values push above the 90th
percentile during the day Monday, and close to the 98th percentile
Tuesday. Meanwhile the operational GFS pushes precipitable water
values to around 1.6 inches Monday, about 12 hours before GEFS
plumes max out for our area. We will focus on an initial upswing
in deeper moisture developing from the south between 03z and 18z
Monday and then see what the week brings us.

At any rate, the first week in October is not likely to be boring.
Also, we may not be done with early transition season tropical
influences with just this one storm.


.AVIATION...Valid through 27/12Z.
Scattered -TSRA/-SHRA across the White Mountains northeast of
KSAD this afternoon and early evening, with perhaps isolated
-TSRA/-SHRA near other mountains east of KTUS. Otherwise, clear
skies to mainly scattered clouds at 8k-12k ft AGL into Wednesday
evening. Surface winds variable in direction mainly less than 12 kts.
Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Scattered thunderstorms mainly across White
Mountains Wednesday afternoon and early evening. Otherwise, dry
conditions through Saturday. Showers and thunderstorms will
return by Monday as moisture increases across the area. 20-foot
winds remain terrain driven at less than 15 mph.






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