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Area Forecast Discussion (AFD)

Issued by NWS Tucson Arizona (TWC)

FXUS65 KTWC 192231

Area Forecast Discussion
National Weather Service Tucson AZ
331 PM MST Thu Sep 19 2019

.SYNOPSIS...Southeast Arizona will see a slight chance of afternoon 
thunderstorms mainly near the mountains east and south of Tucson 
today, along with normal temperatures. Tropical moisture will 
push toward the area over the weekend with shower and thunderstorm
chances as early as Saturday evening. A weather system moving 
through the area Monday and Tuesday could put this moisture to 
good use with increasing rain chances.


.DISCUSSION...A trough pushing through the region will bring drier
air and a few degrees of cooling Friday and Saturday. Big changes
after that. Deep moisture will push toward Arizona from the south
as Lorena and Mario (currently south of Baja) move northward. As
this happens, a Pacific low will dig toward Arizona from the
northwest early in the week. This has the potential for both heavy
rainfall and strong thunderstorm activity.

Lorena is moving quickly northwest away from the west coast of
Mexico and toward the tip of Baja. If she hasen't already regained
hurricane status then she will soon, with early signs of
additional organization on satellite and microwave imagery. 
Immediately to her southwest, Mario is struggling with an area of 
upper level shear and showing signs of poor organization with poor
symmetry and partial exposure of the center. As per NHC, it's 
starting to look like Lorena will continue a stronger push north- 
northwest and may keep separation from Mario.

At any rate, moisture is piling up near the southern Gulf of 
California with over 2.5 inches precipitable water showing up on 
GOES East estimates several hours ago before cloud cover moved in.
Some of that will push northward toward our area over the next 72
hours with a strong moisture increase still expected to begin by 
late Saturday into early Sunday. NAEFS mean specific humidity is
calling for levels near the 98th percentile below 700mb pushing
over the border Sunday, suggesting a moderate to strong surge of 
moisture from the south. We probably won't see 2 inch precipitable
water values past the mid Gulf, but by Monday something on the
order of 1.2 inches in far eastern areas to as much as 1.75 inches
in far western areas. Something like 1.3 to 1.5 inches for Tucson.

The next part of the equation is the trough developing over the
region Monday and Tuesday. A low spinning into the base of the
trough will help drive the weather the first half of the new week,
with a more southerly and slower trajectory bringing better
chances of heavy rainfall as well as the dynamics for strong
thunderstorms. If it's further north and more progressive, then
that limits the window for significant weather this far south. At
this point we will continue to use ensembles and ensemble means.
Essentially that gives us 2 or 3 days of more active monsoon days,
with better potential for nocturnal activity with the low nearby.
Storm total QPF amounts between 1/4 and 1 inch in valleys and 1 to
3+ inches in mountains, with valid arguments for both more and

A westerly flow the second half of the week will push the moisture
back out of the picture.


.AVIATION...Valid through 21/00Z.
A slight chance of -TSRA/-SHRA til 20/04z mainly southeast of a KOLS 
to KSAD line. Otherwise, FEW-SCT clouds (locally BKN) at 8k-12k ft 
MSL. Wind gusts near TSRA to 35 knots. SFC wind less than 12 kts 
thru 20/18Z, with SFC wind direction favoring a SWLY/WLY component 
thru 20/03Z and then variable in direction. Aft 20/18Z, SFC wind 
generally 10-15 kts and favoring a SWLY/WLY direction. Aviation 
discussion not updated for TAF amendments.


.FIRE WEATHER...There will be a slight chance of showers and 
thunderstorms through this evening mainly east and south of Tucson. 
Dry conditions are expected Friday and Saturday. A chance of showers 
and thunderstorms then returns Sunday through early next week as 
deep moisture moves northward from Mexico into southern Arizona. 
Seasonably warm daytime temperatures will occur through Saturday, 
with below normal readings Sunday into the middle of next week. 
Aside from any thunderstorm wind gusts, 20-foot winds will 
generally be terrain driven and less than 15 mph.





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