Day 3 of Rapid City, SD (KRAP)

Last night, it cooled down quickly down to 36F, with the clouds coming in a bit later than expected, allowing more of a cooldown. With mostly cloudy skies today, the daytime warmup was also slower until the clouds cleared for the past 1-2 hours, with a high of 66-67F so far, though it might warm up a bit more before clouds return. Max wind is only 11 kt so far but there could still be bursts of wind later this evening.

 

Here we go with the big storm! Winds will be much stronger than earlier this week, with the surface winds shifting to northeasterly and then northerly. Almost non-stop rain is expected for day 3. Rapid City is situated in a good spot north of a strengthening low pressure, leading to strong synoptic-scale lift enhanced by the upslope easterly flow, leading thick clouds and heavy rain. Just how much rain will fall? The heavy rain bands will be relatively narrow and there will be a tight precipitation gradient near or just north of Rapid City, with the heavy rain making it to around Rapid City but then unable to really push much farther north. It does look like Rapid City will be in the heavy rain area, with the models honing in on the axis of heaviest rain passing through Rapid City or maybe just a tad to the south, with a model average of 2-3” at KRAP. Less will fall if the heavy rain bands stay a little farther south than expected.

 

Temperatures will also be tricky. The high will occur at the beginning (Thursday 06z), with modest cold advection but no real radiational cooling or daytime heating afterward. If it starts raining significantly (like moderate to heavy) before 06z tonight, that will lead to a cooler high (<50F) due to rapid evaporational cooling until saturation having occurred by then. However, most likely, it won’t rain that much before 06z, leading to the air not having saturated yet, and a warmer high. The low will likely occur when the rain is the heaviest, corresponding to the strongest ascent and dynamic cooling, at which point it might mix with snow. The melting snowflakes will lead to additional cooling as the melting process takes up a lot of heat from the air. If it changes over to pure snow, the snow will cause the temperature to drop to 33-34F or so, which sometimes models have a hard time handling. More likely, it will not changeover to snow completely, in which case it would drop to 35-38F or so. Just to the west, a winter storm warning has been issued for Thursday in the Black Hills for heavy snow.

Source: PivotalWeather

Source: PivotalWeather

Source: Meteogram Generator