U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 190531 
Storm Prediction Center ac 190530 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1130 PM CST Mon Nov 18 2019 

Valid 191200z - 201200z 

..there is a marginal risk of severe thunderstorms late this 
afternoon and evening across southern Arizona... 

Strong thunderstorms may impact portions of southern Arizona late 
this afternoon and evening, perhaps accompanied by at least some 
risk for severe hail and wind. 

Within the mid-latitude westerlies across the eastern Pacific, 
models suggest that amplifying ridging will build northeastward 
through British Columbia, the Yukon and portions of the northwest 
territories, taking on more of a positive tilt during this period. 
As it does, the southern portion of a splitting downstream short 
wave trough, and a trailing perturbation, are forecast to dig near 
and west of the Sierra Nevada, and eventually contribute to the 
evolution of a deepening mid-level low near the Southern California 
coast by 12z Wednesday. 

In response to these developments, an initially cut-off mid-level 
closed low, currently still west of baja, likely will be forced 
northeastward. A slow northeastward acceleration toward northern 
baja may be underway by 12z this morning. As it progresses more 
rapidly northeastward, it appears that the low will transition to an 
open wave and perhaps undergo considerable further deformation while 
overspreading southern Arizona and Sonora late this evening into the 
overnight hours. 

Farther downstream, beneath building larger-scale ridging across the 
southern and Central Plains into the lower and middle Mississippi 
Valley, a southerly return flow may gradually commence across the 
lower Rio Grande Valley into portions of the Southern Plains, in the 
wake of large-scale troughing beginning to shift east of the 
Atlantic Seaboard. 

While convective potential is expected to remain negligible across 
most areas east of The Rockies, an influx of moisture from the lower 
latitude eastern Pacific, and through the Gulf of California, 
probably will contribute to destabilization supportive of scattered 
thunderstorm activity across the southwest. This may include 
portions of the Colorado plateau and southern rockies into southwest 
Texas, in addition to much of central and southern Arizona. 

..central and southern Arizona... 
On the leading edge of the moisture return, increasing forcing for 
ascent, perhaps aided by a mid-level perturbation pivoting around 
the approaching closed low, may support considerable convection and 
embedded thunderstorms across parts of northern Sonora into southern 
Arizona by 18-21z. This activity is expected to be generally weak 
with negligible severe risk, and may tend to diminish while 
spreading toward the Mogollon Rim. 

In its wake, boundary layer moistening and breaks in overcast may 
allow for the development of weak to moderate cape across Sonora 
into at least the southern Arizona border vicinity. This may become 
sufficient to support vigorous thunderstorm development, 
particularly near the leading edge of the northeastward advancing 
mid-level dry slot, where steepening mid-level lapse rates may 
coincide with sufficient strengthening of deep-layer shear to 
support isolated supercells. While strongest cells may initiate 
across Sonora, activity may spread across the international border 
into Arizona with at least some continuing risk for severe hail and 
locally strong surface gusts, mainly during the 20/00-03z time 

.Kerr/Cook.. 11/19/2019 


Mesoscale Discussion

acus11 kwns 151935 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 151934 

Mesoscale discussion 2149 
National Weather Service Storm Prediction Center Norman OK 
0134 PM CST Fri Nov 15 2019 

Areas affected...northern and western ME to the Champlain Valley 

Concerning...heavy snow 

Valid 151934z - 152300z 

Summary...snow squalls should push east-southeast from the Saint 
Lawrence valley, most likely across portions of northern and western 
ME between 4 to 7 PM EST. A brief period of heavy snow with 0.5-1.0" 
in 30 min is likely. 

Discussion...19z surface analysis placed a cold front across the 
Saint Lawrence valley. Canadian radars in Montreal and Quebec city 
have sampled a snow squall along the front. Somewhat enhanced 
reflectivity was noted along the eastern portion of the squall near 
Quebec city, which is consistent with a recent increase of 
low-topped convective development/colder cloud tops in visible and 
infrared satellite imagery. Recent amdar data near Montreal sampled 
inversion heights to 750 mb and near Quebec city to 700 mb. Latest 
rap/NAM guidance both are consistent with inversion heights further 
deepening as the shortwave trough and attendant cooling aloft 
approaches from southwest Quebec. Lack of stronger convergence along 
the front, in addition to a modest surface rise/fall couplet 
behind/ahead of the front, does suggest that squalls which progress 
across the international border will probably subside within a 
couple hours after sunset. 

.Grams.. 11/15/2019 

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


Latitude...Lon 45347358 45757143 47606922 47386776 47036763 45686894 
44617101 44057257 43947341 44197411 44847428 45347358