U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 211616 
Storm Prediction Center ac 211615 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1115 am CDT sun Oct 21 2018 

Valid 211630z - 221200z 

..there is a marginal risk of severe thunderstorms late this 
afternoon across parts of central and eastern Arizona... 

A couple of strong storms may impact portions of central and 
southern Arizona this afternoon, accompanied by some risk for severe 
hail and locally strong wind gusts. 

Within the main branch of westerlies emanating from the mid-latitude 
Pacific, amplified large-scale ridging appears likely to rebuild 
across western Canada and the Pacific northwest/northern 
intermountain region/rockies. Downstream troughing east of the 
plains will persist, but the cyclonic regime is expected to become 
less amplified, as a strong embedded short wave trough continues 
accelerating east-northeast of the northern mid Atlantic/southern 
New England coast. 

Models indicate that a significant cold front associated with the 
short wave will advance southward through the remainder of the 
Florida Peninsula/Keys by 12z Monday. However, the southwestern 
flank of the frontal zone is forecast to stall and return northward 
across the western Gulf of Mexico, and possibly into parts of deep 
south Texas. This is expected as broad upper troughing, and an 
embedded mid-level cyclonic circulation, gradually progress inland 
of the California coast, within a belt of westerlies emanating from 
the southern mid-latitude/subtropical eastern Pacific. 

..Great Basin/southwest... 
Moisture return on southerly low-level flow has contributed to 
sufficient destabilization for scattered ongoing thunderstorm 
activity across parts of southern Nevada/northwest Arizona and 
southwestern Utah. This has been supported by forcing for ascent 
associated with one short wave impulse embedded within the 
large-scale troughing, now progressing northward into the Great 

In its wake, another perturbation appears in the process of 
progressing northeastward toward the lower Colorado valley. Models 
suggest that this could be accompanied by initiation of scattered 
thunderstorm activity across parts of central/southern Arizona as 
early as mid afternoon. This is where/when higher low/mid-level 
moisture content (characterized by precipitable water up to around 1 
inch) may contribute to cape up to around 1000 j/kg, beneath at 
least modestly steep mid-level lapse rates. 

Lower/mid-tropospheric wind fields appear likely to remain rather 
modest to weak in strength. However, perhaps aided by 50+ kt 
southwesterly flow in the 300-250 mb layer, shear through the depth 
of the convective layer will be strong, and may contribute to 
potential for sustained/organized storms accompanied by at least 
some risk for severe hail and locally strong surface gusts through 
early evening. 

.Kerr/karstens.. 10/21/2018 


Mesoscale Discussion

acus11 kwns 201608 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 201608 

Mesoscale discussion 1588 
National Weather Service Storm Prediction Center Norman OK 
1108 am CDT Sat Oct 20 2018 

Areas affected...Southern Lower Michigan 

Concerning...severe potential...watch unlikely 

Valid 201608z - 201815z 

Probability of watch issuance...5 percent 

Summary...strong damaging gusts (40-50 mph) are possible over the 
next several hours as a broken convective line with embedded cells 
moves east across Southern Lower Michigan. 

Discussion...visible satellite imagery shows a cumulus field 
developing over Southern Lower Michigan where surface temperatures 
are rising into the middle 50s degrees f. Radar imagery shows a 
low-topped convective line with embedded cells showing weak 
organizational characteristics located in a south-southwest to 
north-northeast line immediately east of Grand Rapids. As a potent 
mid-level shortwave trough continues to pivot southeast over the 
central Great Lakes this afternoon, large-scale forcing for ascent 
will maximize coincident with peak heating. 

Modifying rap soundings over Southern Lower Michigan for middle 50s 
surface temperatures yields a nearly 9 degree c/km 0-3 km lapse rate 
because of very cool mid-level temperatures (-25 degrees c at 500 
mb). Although winds in the lowest 3 km are 40 kt or lower per kgrr 
VAD data, it seems plausible as the convection slowly organizes, 
potential for strong gusts via horizontal momentum transport may 
occur. The main threat with the strongest storms is localized wind 
damage due to 40-50 mph gusts. 

.Smith/guyer.. 10/20/2018 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 42988486 43008346 42568268 41878309 41788398 42048531