Tuesday, July 14, 2009

Thunderwear Recommended

Monday Almanac for the Twin Cities. High: 82, Low: 60. Precipitation: 0" (35 degree low at International Falls!)

Paul's Outlook

Today: Mostly cloudy, a little more humid. Growing risk of showers/T-storms later in the day, a small percentage may be severe. Winds: S/SE 10-20 (higher gusts in T-storms). High: 77

Tonight: Lingering showers, a few heavier thunderstorms. Low: 63

Wednesday: Drier, partly sunny with a stiff breeze. Winds: NW 10-30. High: 76

Thursday: Mix of clouds and sun, breezy and cooler as a reinforcing cool front arrives. High: 74
(holding in the 60s up north)

Friday: More sun, less wind, still on the cool side. High: 75

Saturday: Sunny and warmer, a beautiful day! High: 81

Sunday: Fading sun through high clouds, good lake-day. High: 83

Monday: Partly cloudy and seasonably warm. High: 85

Tuesday: Growing chance of showers, few T-storms. High: 82

Today will be unsettled with more clouds streaking overhead, and a growing risk/opportunity of bumping into something thundery and wet as the day goes on. Not all of the ingredients are present for a widespread severe weather outbreak, but there may be just enough moisture and instability for rising thermals of warm air to break through a weak inversion "cap", accelerating upward into flat-topped, anvil-capped cumulonimbus thunderstorms. The entire state is under a "slight risk" of isolated severe storms.

                        Station ID: KMSP Lat:   44.88 Long:  -93.22                                                      
NAM Model Run: 0Z JUL 14, 2009

Forecast Hours: 0hr 6hr 12hr 18hr 24hr 30hr 36hr
Sfc Prs(mb): 987.2 988.2 985.5 982.2 978.3 978.2 978.7
Mean SLP (mb): 1016.7 1017.8 1015.5 1011.5 1007.5 1007.5 1008.4
2m agl Tmp (F): 76.9 62.6 64.8 74.2 69.5 69.2 66.4
2m agl Dewpt(F): 46.6 47.2 50.2 63.9 67.2 68.7 64.6
2m agl RH (%): 33 56 58 69 92 98 94
10m agl Dir: 102 121 147 168 145 200 273
10m agl Spd(kt): 4 7 11 15 16 8 10
6hr Precip (in): 0.00 0.00 0.00 0.02 0.40 0.04 0.00
AccumPrecip(in): 0.00 0.00 0.00 0.02 0.43 0.47 0.47
Sfc CAPE (J/kg): 0.0 0.0 0.0 0.6 44.6 838.8 89.3
Sfc CINH (J/kg): -0.5 0.0 -0.6 -13.9 -14.3 -51.2 -160.7
0-3km Hel(J/kg): 90.2 143.3 181.8 221.1 541.5 194.2 81.8

NAM model run from Monday evening, with a 24 hour prediction showing a high "helicity" value for 7 pm this evening (541.5) suggesting strong wind shear aloft, enough to get a few isolated thunderstorms "spinning", which may in turn lead to large hail, even an isolated tornado.

Meteorologists looks at many factors when gauging the risk of severe weather, including something called "helicity", which is the potential for thunderstorms to spin, rotate. This is what Doppler Radar does so well: it detects mutating thunderstorms that spin; these are the (rare) storms most likely to sustain warm updrafts over the course of many hours. Usually a thunderstorm self-destructs, rain and hail-cooled air snuffs out the warm updraft and the storm fizzles. But when there is considerable "wind shear" (changing wind speed and direction with altitude) thunderstorms can spin, in essence protecting the warm updraft, sustaining a storm over the span of many hours, generating enough "vorticity" (spinning energy) to spin up nature's most violent updraft: the tornado. Large hail is often a strong tip-off that a storm updraft may be severe enough to spin up a tornado. Anything larger than golfball-size gets my attention. Most tornadoes are usually preceded by a few minutes of large hail. Skies often brighten, rain and hail tapers, it almost looks like the storm is over, but then, at the rear of the storm, in the southwestern quadrant, a rotating "wall cloud" lowers to the ground. It's here that a tornado is most likely to form, at the tail-end of a hail-producing, rotating thunderstorm.

Amazing file photo of a dramatic wall cloud, with obvious strong rotation. Here is where a tornado is most likely to form.

One of the problems with Doppler Radar: only 20-30% of all spinning thunderstorms go on to spawn a tornado; it's almost impossible to determine WHICH rotating storms will go on to drop a tornado, so as a result the NWS tends to over-warn when it comes to tornadoes. Many times they'll issue a warning based on just Doppler returns. Their confidence level goes up dramatically if they receive confirmation from professional storm spotters on the ground, specially-trained SKYWARN spotters who communicate in to local Civil Defense and National Weather Service employees. It's great to have Doppler data, but only a human being, literally out in the field, can CONFIRM that a given, spinning storm is actually giving birth to a deadly tornado. BTW the NWS is always looking for more SKYWARN spotters - they give special classes and if you have a strong interest in weather and want to volunteer in your community you should look into it. It's a great group of people (all weather fanatics!) and these men and women are on the front-line, the eyes and ears of the National Weather Service, an integral part of the warning process that saves so many lives every year.

To go to SKYWARN's local home page and learn more about the organization and upcoming classes click here.

Latest SPC Outlook for Tuesday. There is a 30% probability of severe weather (1" diameter, quarter-size hail and/or wind gusts > 58 mph) within 25 miles of any location within the red hashed area.

WRF/NMM Model Output for 7 pm this evening. The models can't detect individual thunderstorms or even delineate precisely where bands of storms will set up. In this kind of a convective, showery pattern some towns/farms may pick up as much as 1" of rain, while 5 miles away nearby neighborhoods see little more than .05" of precipitation. As an average I think many of us will see anywhere from .20 to .35" of rain later today and early tonight.

500 MB Winds for 7 pm Monday evening. This graphic shows wind speeds roughly 18,000 feet above the ground, outlining the "jet stream" (shaded in green). The yellow regions show the fastest winds within the jet core, what meteorologists refer to as "speed maxes". One of these speed maxes passing overhead can create an environment where storms are more likely to ignite late in the day. Overall this is a pattern more typical of mid or late September than mid July, the jet core some 200-400 miles farther south than usual for mid summer. Why? Great question - wish I had a great answer.

No comments:

Post a Comment