Thursday, April 27, 2017

April to end wet, then a cool start to May

With the first in a series of storm systems behind us (last night's semi-bust [at least locally] of a perfectly good Enhanced/Moderate Risk scenario), we turn our attention to the last weekend in April, one in which there are many outdoor activities planned, including a Redbirds home-stand and at least a couple of civic festivals. Fortunately, the forecast has improved slightly from a few days ago, at least for part of the weekend.

Preliminary storm reports from April 26 overlaid on the morning SPC outlook. While there were about 200 wind and hail reports for the day, except for a string of reports that extended well beyond the Slight Risk area in the Midwest, reports were fairly scattered and there were zero tornado reports, the basis for the Moderate Risk. (NOAA/SPC)

Friday: Gulf opens up

After a pleasant spring day on Thursday with highs near 70 and comfortable humidity, Wednesday night's front returns north on Friday morning as a warm front. The passage of the front means the arrival of humidity levels we haven't seen yet this spring as dewpoints climb to near 70 degrees or a bit higher. You'll notice the sticky-ness when you leave work or pick up the kids in the afternoon, as well as the return of gusty south wind signaling that the Gulf is open for business.

A couple of showers are possible during the morning as the front surges north and low pressure starts traveling along the front to our west, then there's a chance of a few thunderstorms in the afternoon, though models differ on that prediction. If they do develop, there will be a good deal of instability so gusty wind and pockets of hail are possible in any strong storms. High temperatures reach the lower 80s, and if the sun peaks through the clouds some in the afternoon, perhaps mid 80s.

Convective Available Potential Energy (CAPE, or instability, aka "storm fuel") forecast by the GFS model on Friday evening will be very high - more than enough to fuel strong storms, IF they form.

Friday Night: borderline

The fireworks really start along the front on Friday evening as thunderstorms explode to our north. A corridor from OK across northern AR into the Ohio Valley will see multiple rounds of storms with heavy rain Friday night. The metro is right on the southern edge of where rain will fall and chances vary based on the model scenario you want to buy. Northern areas definitely have a better shot at rain than north MS, but we could all stay dry too. For now, we'll carry a generic "chance of thunderstorms" until it becomes a little clearer. The corridor to our north will not only be prone to some flooding, but also severe weather, including large hail and damaging wind.  An Enhanced Risk (category 3/5) is outlooked as far south as I-40. Our threat locally is conditional on storms actually occurring. Once again, if they happen, they could be strong. Temperatures won't drop below 70 degrees Friday night.

The severe weather risk is Enhanced (3/5) for areas north of I-40 on Friday night. If storms form this far south, they could be severe. Chances of them forming in the metro are about 30-40%. (NOAA/SPC)

Saturday: summertime

Saturday may be our first taste of early summer as the front remains well to our north and gusty southerly wind indicate the Gulf is still open, leading to continued high humidity. With some sunshine expected, temperatures that start near 70 will soar into the 80s and perhaps close to 90 in spots and the term "heat index" may be bantered about as it will likely eclipse 90 for the first time in 2017. With the heat and humidity in place, there appears to be a chance of some afternoon "peak heating" thunderstorms not atypical of a Mid-South summer day. If storms form, they could again be strong with high wind gusts the main threat. Outside of storm gusts, general wind will be very gusty throughout the day as well, peaking at 25-35 mph and perhaps higher.  If you have outdoor plans, keep them. If you have outdoor objects, secure them. If you're a vendor at a festival, leave the brochures at home.

The European model, which we rarely show, highlights the potential for strong wind gusts on Saturday, perhaps approaching 40 mph at their peak. (WxBell)

Sunday: wet

The weekend ends with a very wet, windy, and potentially stormy day. The system to our northwest finally gets pushed back to the southeast by low pressure that moves out of the southern plains. Rain starts early, maybe before dawn, and continues for several hours. The GFS and European models differ on when the front arrives to shut off the spigot, but it will likely be no earlier than mid-afternoon and possibly as late as the evening. Rain could be heavy with 2-3" possible during the day.

The Weather Prediction Center forecasts rainfall for the entire weekend in the 2-4" range in the metro, but over 6" not too far away in AR and MO. Expect this to lead to additional rises in local and main stream rivers like the Mississippi into next week.

In addition, there are indications that it could be one of the windiest days in a long time, especially if the front lingers to the west until late in the day. Gusts have the potential to reach 40-50 mph, perhaps stronger in storms that are also expected. Saturated soil and strong wind could be enough to down some trees. Stay tuned, as the models still need to come to some consensus.

Next week: below normal temps

Once the big system moves out Sunday night, spring returns early in the week with dry air, cooler mornings, and pleasant afternoons. One more system that brings a chance of rain mid-week will be followed by an even cooler spell, just in time for Beale Street Music Fest! Let's hope for a great weekend for the massive party on the river!

In the meantime, Go Grizzlies!!

Erik Proseus
MWN Meteorologist

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