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Area Forecast Discussion (AFD)

Issued by NWS Tucson Arizona (TWC)

FXUS65 KTWC 231543

Area Forecast Discussion
National Weather Service Tucson AZ
843 AM MST Mon Sep 23 2019

.SYNOPSIS...Showers and thunderstorms will be on the increase today 
into Tuesday as a low pressure system passes through and interacts 
with tropical moisture over the area. Some strong to severe 
thunderstorms will locally heavy rainfall can be expected today and 
Tuesday. Rain chances will continue Wednesday into Thursday followed 
by dry conditions Friday into the weekend. Day temperatures will 
remain at or slightly below normal this week. 


.DISCUSSION...It will be a very dynamic 36 hours across central and
southern Arizona, with the first wave already having occurred NE of 
Phoenix with flooding rains earlier this morning. I did a quick update
this morning for the following. Trimmed pops for the first 2/3rds of
the day for areas south and east of Tucson as it will be relatively 
quiet this area today. Added enhanced weather wording for potential 
severe thunderstorms later this afternoon into the evening northwest 
of Tucson. Conditions are ripe for central/southwest Arizona with 
significant CAPE/Shear and moisture for severe storms that may clip 
the NW portions of our forecast area. Then for tonight ramped up POPS
and rainfall amounts for late tonight through Tuesday morning and
added enhanced heavy rain wording. Beginning to look like a 
significant rain event developing after midnight tonight over parts
of southeast Arizona that will continue well into or through Tuesday
morning. Some locations (especially mountains) may see inches of 
rain during this time. Flash Flood Watch already in effect for this 
period of time which looks good. 

In support of SPC/WPC and our operations, additional soundings will 
be done at 18Z today, 06Z tonight, and 18Z Tuesday. 


.AVIATION...Valid through 24/12Z.
A few isolated -TSRA/-SHRA will be possible this morning. Scattered 
TSRA/SHRA is expected to develop late this afternoon and become more 
widespread through the evening and overnight hours. Some storms could
be strong to severe with hail and strong wind gusts. Ceilings will 
generally be SCT-BKN 4k-8k ft MSL with MVFR and even occasional IFR 
ceilings in and around thunderstorms. Surface winds this morning 
variable less than 10 kts becoming southwesterly/westerly this 
afternoon and evening at 8-12 kts. Near thunderstorms outflows will 
cause winds to be variable in direction with gusts to 30-40 kts 
possible. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Isolated showers and thunderstorms are possible 
this morning. Rain chances increase this afternoon with widespread 
showers and thunderstorms expected Monday evening into Tuesday ahead 
of an approaching low pressure system. Locally heavy rainfall may 
occur with gusty winds at times. Scattered thunderstorms to continue 
Wednesday into Thursday which will result in periods of wetting 
rains across nearly all of the forecast area. Dry conditions are 
then expected Friday through the coming weekend. 20-foot winds will 
be terrain driven less than 15 mph except in the vicinity of 
thunderstorms where gusty and erratic winds will be possible. 


.PREV DISCUSSION... /issued 355 AM MST Mon Sep 23 2019/ 
Deeper moisture continued to spread into the region as an upper low 
over Nevada drops south into Arizona later today. Large-scale ascent 
associated with the upper low in combination with this moisture and 
an unstable air mass will translate to concerns for strong to severe 
thunderstorms with very high rainfall rates. Isolated to scattered 
showers and thunderstorms will develop this morning across mainly 
areas north and west of Tucson. This activity will become more 
numerous this afternoon and tonight across southeast Arizona. A brief
lull or decrease in showers and thunderstorms may occur in some 
areas Tuesday morning, but an upswing in this activity can be 
expected Tuesday afternoon. That said, a Flash Flood Watch has been 
posted for most of southeast Arizona for mainly this evening into 
Tuesday evening. Please refer to the AWIPS header PHXFFATWC or WMO 
header WGUS65 for more details. The upper low will become centered 
over southwest Arizona/northern Baja California Tuesday and Wednesday
before lifting northward and exiting the state on Friday. Improving 
weather can be anticipated Thursday with dry conditions prevailing 
late in the week. High temperatures will remain near or slightly 
below normal this week. 


Flash Flood Watch from 6 PM MST this evening through Tuesday 
evening for AZZ503-504-507>509-511>515.

Flash Flood Watch through Tuesday evening for AZZ502-505-506.




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