Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 181538

Area Forecast Discussion
National Weather Service Tucson AZ
838 AM MST Sun Mar 18 2018

.SYNOPSIS...A weather system will bring a chance of valley rain
and mountain snow mainly this morning. Cooler temperatures into
tonight before high pressure brings a warming trend the first half
of this week. Another system will brush by mainly north of the area
later in the week.


.DISCUSSION...A storm system over northern Arizona this morning
will continue to push eastward into the 4-corners area by mid day.
An associated cold front is just west of Tucson at 8:30 AM, with
light showers now developing just ahead of and with the frontal
boundary. As showers spread into the Metro area from the
southwest, precip amounts are expected to remain on the light
side (near or under .05 inches in the valleys). Mountain snow in
the 1 to 3 inch range, with the high end of that northeast of
Tucson in Graham and Greenlee counties. The timing of the front
will mean warmest daytime high temperatures east of Tucson. Our
current forecast handles all of this well, so no updates this
morning. For additional details, please see the previous
discussion below.


.AVIATION...Valid through 19/18Z.
Isolated to scattered -SHRA and mountain -SHSN in a band from
southwest of KTUS northeastward to the White Mountains northeast
of KSAD this morning. Isolated -SHSN vicinity White Mountains
this afternoon ending by 19/00Z. Otherwise, SCT-BKN clouds 5k-9k
ft AGL KTUS vicinity northeastward this morning clearing from the
west this afternoon. Surface wind sly to wly 5-15 kts this
morning then wly/nwly 10-22 kts with gusts near 30 kts this
afternoon. Strongest speeds east of KTUS. Decreasing winds this
evening then variable in direction less than 10 kts late
tonight/early Monday morning. Aviation discussion not updated for
TAF amendments.


.FIRE WEATHER...Expect isolated to scattered showers producing
light valley rain and mountain snow amounts mostly in a band from
southwest of Tucson northeastward to the White Mountains today. Dry
conditions will then prevail tonight through Wednesday night. A few
showers may develop Thursday west of Tucson followed by a slight
chance of showers area-wide Thursday night into early Friday. Dry
conditions will return by next weekend. Gusty west to northwest
winds this afternoon will diminish this evening. 20-foot winds will
mainly be less than 15 mph Monday into Wednesday. Some gusty
southwest afternoon winds should return Thursday into Saturday.


.PREV DISCUSSION...A deep upper trough was over much of the
western CONUS early this morning with upper ridge axis just off
the West Coast, and a north to south ridge axis near the
Mississippi River Valley. Mid/upper level trough axis and
associated cold front extended from western Utah southward into
southwest Arizona at this time. The 18/00Z NAM12/GFS/ECMWF/CMC
remained consistent versus their respective previous solutions
with depicting light valley rain and mountain snow amounts to
occur this morning.

For this forecast issuance, have maintained isolated to scattered
valley rain/mountain snow showers this morning generally from
near Kitt Peak northeastward across the Tucson metro area, and
further northeast toward the White Mountains. Dry conditions will
occur this morning across western Pima County, and should occur
across southeast Cochise County. The mid/upper trough axis and
cold front is progged to move rapidly eastward into
central/eastern New Mexico this afternoon. Stronger westerly flow
aloft will lead to a rapid ending of precip this afternoon.
Moisture should be sufficient for isolated snow showers across the
White Mountains this afternoon then ending by this evening.

The model solutions have trended downward with liquid/snow amounts
versus 24 hours ago. Thus, expect valley and mountain rainfall
amounts to be mostly less than 0.10", and snow accumulations of just
an inch or perhaps two inches across the higher White Mountain

A dry regime will prevail tonight into Wednesday, and perhaps across
much of the area Thursday. Upper ridge aloft near the West Coast
Monday will approach the area from the west Tuesday, then continue
eastward across southeast Arizona Wednesday. Southwesterly flow
aloft will strengthen Thursday ahead of the next deepening upper
trough near the Pacific Northwest extending southward adjacent the
California Coast.

Regarding precip potential for Thursday into Friday, the
deterministic ECMWF remained the preferred solution versus the
somewhat wetter/faster GFS, and the slower CMC solution. There is a
slight chance of showers Thursday afternoon west of Tucson, then a
slight chance of showers area-wide Thursday night. A slight chance
of showers continues Friday morning generally from Tucson
northeastward to the White Mountains, then dry conditions to return
Friday afternoon into Saturday under drier westerly flow aloft.

High temps this afternoon will be 5-10 degrees cooler versus
Saturday, with the least amount of cooling southeast of Tucson. A
strong warming trend will occur Monday through Wednesday. Appears
Wednesday should be the warmest day, and have trended temps slightly
higher primarily based on local thickness studies. A gradual cooling
trend should then occur Thursday into Friday followed by no
significant change in high temps Saturday.






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