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Area Forecast Discussion (AFD)

Issued by NWS Tucson Arizona (TWC)

                            
000
FXUS65 KTWC 201634
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
934 AM MST Fri Sep 20 2019

.SYNOPSIS...Tropical moisture will push toward the area over the 
weekend with shower and thunderstorm chances near the border as 
early as Saturday afternoon. A weather system moving through the 
area Monday and Tuesday will interact with this moisture for 
increasing chances of rain, and possible heavy rain. Temperatures
will remain near seasonable readings the next several days.

&&

.DISCUSSION...As a trough pushes through the region north of our
area this morning, a sustained westerly flow has dried us out.
Precipitable water on the 12Z KTWC sounding was down to 1/2 inch
and surface dew points have dropped into the 40s this morning.
This will change a lot over the next 48 hours as TS Lorena pushes
deep moisture toward our area. Precipitable water values are up to
1.9 inches across the central Gulf of California and southern
half of Sonora this morning. We should see the first of some of
that moisture by late Saturday, with increasing light to moderate
shower chances near the border. 

Over the past 24 hours, Lorena has gained some separation from
Mario and may actually be able to be resolved on her own. Current
NHC track puts her weakening along the Baja coast as she
interacts with the mountains, pushes into cooler water, and 
encounters increasing shear. It's not out of the question that the
remaining mid level circulation will at some point decouple and 
shear into the higher latitude flow and into the desert southwest 
late in the weekend. However, the significance of this event for 
SE AZ is still going to come down to the timing and trajectory of 
the trough and associated low approaching the area from the 
northwest Monday-Tuesday. No updates this morning as we continue 
to favor a forecast with heavy influence from ensembles and 
ensemble means. This suggests an elevated period of heavy rain 
and/or strong thunderstorm chances primarily Monday and Tuesday.

&&

.AVIATION...Valid through 21/18Z.
Expect mostly clear skies this morning with FEW-SCT clouds 8k-10k
ft MSL this afternoon and evening. Surface wind variable less 
than 10 knots becoming southwesterly at 10-14 kts with gusts to 20
kts this afternoon and early evening. Winds then calm down again 
after sunset. Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Expect dry conditions today then a slight chance of 
showers and thunderstorms along the international border Saturday 
afternoon and evening as moisture begins to move in. Deeper moisture 
moves in starting Sunday leading to a chance of showers and 
thunderstorms area wide Monday and Tuesday. Wednesday into 
Thursday storm chances decrease and stay mainly from Tucson 
eastward. Seasonably warm daytime temperatures will occur today, 
with below normal readings Saturday 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.

&&

.PREV DISCUSSION...Mostly clear today with a few afternoon clouds
over the mountains developing this afternoon. The main focus will
be on the rain potential this weekend into early next week as 
deeper moisture spreads northward into the region. Models were in 
good agreement with moisture associated with tropical storm Lorena
spreading northward into Arizona, especially by Monday as a 
system deepens over the western United States. This system will 
drop farther south and track through Arizona by late Monday into 
Tuesday. Large scale ascent associated with this system combined 
with the deeper moisture will result in area wide showers and 
thunderstorms early next week. The latest Weather Prediction 
Center day 3 excessive rainfall outlook placed portions of central
and southern Arizona in the marginal risk category. This 
excessive rainfall potential will be something to follow closely 
as Monday approaches. Otherwise, near or slightly below normal 
high temperatures can be anticipated through next week.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

Meyer

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