Sunday, April 10, 2016

Possible severe storms Monday, then cool... then warm!

A chill to start the weekend turned very windy and much warmer today as a developing weather system in the Plains begins to exert its influence on the region. In fact, with the dry, tilled ground in the Mississippi Delta and southerly wind gusts of 40-50 mph, blowing dust has resulted in very low visibility in areas of northeast AR and the National Weather Service took the unusual step of issuing an Arizona-like Dust Storm Warning for far northeast AR!

At least a couple of accidents are blamed on visibility that dropped below 1/2 mile in that region. Here in the immediate metro, the dust is not a serious issue, but looking through the haze to the west might mean a nice sunset to an 80-degree day!

Turning our attention to the week ahead, most of the action will be early in the week (Monday in fact) followed by another cool spell, then gradual warming with temperatures well above average to end the week. After Monday, rain chances are slim to none the rest of the week, except perhaps on Thursday when showers or a thunderstorm could affect mainly north MS.

On Monday, we will start the day with mild temperatures as lows remain in the 60s overnight due to increasing moisture and clouds and continued (but not quite as strong - gusting 25-30 mph) south wind. Chances of showers increase during the morning as temperatures climb towards 70° and south wind remains gusty. By afternoon, thunderstorms will develop to our west and begin moving into the metro ahead of a developing area of low pressure in the southern Plains. A few factors will combine to create an atmosphere capable of strong storms and heavy rain over the metro.

The nearly two days of strong southerly wind that precedes the system will result in dewpoints (a measure of humidity in the air) that reach the lower to mid 60s (plenty to feed any storms - see graphic below). The combination of available moisture and surface temperatures that reach the lower 70s will create instability in the atmosphere as a cold front approaches from the north. The instability, measured by an index called CAPE (Convective Available Potential Energy - or energy available to create convection/storms), will likely rise to near 1000 by late afternoon, especially along and south of I-40. The highest CAPE values, where instability is strongest, will be across southern AR (see second graphic below).

Forecast dewpoints at 4pm Monday from the Global Forecast System (American/GFS) model. Mid 60s is plenty of "juice" for the formation of thunderstorms. The drier air (lower dewpoints) to the north will be moving south by evening behind a cold front that marks the edge of the drier air. Graphic courtesy Pivotal Weather.

Forecast CAPE (instability) from the GFS model at 7pm Monday shows the best "fuel" for storms well to the southwest, but sufficient instability to promote thunderstorms in the metro. As the front approaches from the north, instability is shut off. Graphic courtesy WxBell.

Because of the high instability values, strong wind fields, an approaching low pressure system over southwest AR into the ArkLaTex region, an Enhanced Risk (category 3/5) of severe weather is present in that region. However, a Slight Risk (category 2/5) is present eastward up to the southwest corner of the metro, while a Marginal Risk (category 1/5) covers the metro (see graphic below). In other words, the highest probability of severe weather is well to our south and west (see second graphic below), but a few strong to borderline severe storms are possible in our area. The main risks will be large hail and damaging wind from late afternoon through the evening. The tornado threat is very low.

The Storm Prediction Center severe weather outlook for Monday shows the highest probability of severe weather where the greatest instability is - southwest AR. A Marginal Risk (category 1/5) exists for the metro with a few strong storms possible from mid-afternoon through early evening.

The probability of severe weather within 25 miles of a point on Monday is shown in this graphic from SPC. The metro is within a 5% risk area, which is relatively low, though not far from the 15% area. This could change on Monday.

A factor that limits our chance of severe weather includes the front pushing into the area before the main dynamics of the storm arrive from the west. Once the front arrives and wind shifts to the north early in the evening, the instability is shut off and the strong storm environment ceases to exist. Should the front slow up a little bit or afternoon rain fail to materialize, then instability could be stronger and our severe weather risk could climb. This forecast is the best educated guess based on the predicted state of the atmosphere about 24 hours out.

Besides severe weather, the atmosphere will have plenty of available moisture to work with as the system arrives, so heavy rain is also a good possibility (which will also end the blowing dust and, temporarily, the pollen, problem). Forecast rainfall amounts vary fairly widely between models due to placement of storms, but widespread 1.5-2" readings are expected from afternoon into the evening with some areas well above 2" if storms train over any particular area. In addition, rain will continue once the front passes tomorrow evening, so some heavy rain will be possible through perhaps midnight.

The National Weather Service forecast for rainfall amounts through Tuesday morning shows the heaviest over north MS closer to the track of a low pressure system, though models differ on exact amounts, ranging from just over an inch to well over 2". Graphic courtesy WxBell.

Following Monday night's rain ending, we appear headed for a mostly dry, but cooler, spell as we head into mid-week. Highs will be back down in the 60s with lows in the 40s. A disturbance passing by to the south will bring clouds and perhaps a few showers or a thunderstorm Thursday, mainly over north Mississippi, before we begin a warming trend that takes us into next weekend. I don't think we'll need the clothing layers next weekend that we started this weekend with! the GFS model forecast highs and lows for the next 10 days are shown below and are provided for planning and guidance only. Average highs/lows for this time of year are in the lower 70s and lower 50s, respectfully.

GFS Ensemble forecast temps for the next 10 days, courtesy WxBell. Temps are not exact and are to be used for trend analysis.

Stay tuned to our social media feeds and mobile apps tomorrow for the latest on severe weather trends and any severe weather watches or warnings that may be issued. Links to download the app or monitor our social media feeds are provided below.

Erik Proseus
MWN Meteorologist

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