heavy_wx

The transition to sleet here in southern MoCo lines up pretty well with the low RhoHV and high reflectivity on the 4-degree PPI scan from LWX.

Just changed over to sleet here in North Bethesda.

There is likely some partial melting of the aggregates, ahead of the more visible reduction in correlation coefficient, causing the ice particles at the surface to become more compact. A mix of fully melted liquid drops and partially melted ice particles is typically needed to produce the observed reductions in correlation coefficient within the melting layer.

I measured just over 4" of snow here in North Bethesda with moderate snow falling currently. The aggregates are mostly compact indicating less than optimal snow growth at temperatures > -10C and possibly some riming. I did see a few dendrites mixed in so there is still some growth occurring at temperatures < -12C.

Measured about 5" of accumulation here. There are currently some large aggregates falling within the region of 30+dBZ reflectivity in southern MoCo.

Interesting signature for banding in the 06z GFS (and 06z 3-km NAM) around 12z-18z Monday, just as the dry air starts to work in above 600 mb. This dry air above the lower-level moisture produces convective instability above the frontal surface between 750-650 mb south of the frontogenesis max around 800 mb. This instability would favor vertical motion and ice crystal generating cells just to the north of the approaching dry air. Given the temperatures > -12C and the likely presence of supercooled water, graupel is a definite possibility within this area of precipitation.

We are within the ~25-dBZ reflectivity band here and getting steady light snow with mostly small aggregates.

We're getting some flurries here now.

Saw the bright pink colors here in southern MoCo.

Getting some melting snow here in the heavier bursts of precip.

Wet snowflakes just starting to mix in here.

Snow mixing with sleet now in North Bethesda.

Aggregates starting to get bigger here as the temperatures warm aloft and the band of heavier snow approaches.

Flurries here in North Bethesda.

Some flurries now in North Bethesda.

39.14N, 76.85W

Moderate-sized aggregates falling now.

Light snow with small flakes here.

If the clouds do hold off ahead of the cold front, it could be one of those days where the models underestimate surface solar radiation (and thus surface temperatures) owing to the lack of foliage this time of year.

Moderate snow with wet aggregates here.

Pretty much all sleet here in North Laurel.

This event is a good example of why these "precip-type" radar maps are often flawed. The indicated precip type is not based purely on radar data; they add supplemental model data to determine the likeliest precip type at the location of each radar observation. The dual-polarization observations, however, show the sleet/snow line from southern Carroll County northeastward into PA, where reflectivity (ZH) and differential reflectivity (ZDR) values are enhanced due to bright banding, and correlation coefficient is reduced due to a mix of melting particles with different shapes. SImilarly, the rain/freezing rain transition is where the correlation coefficient increases south of Anne Arundel County, with both ZDR and ZH values decreasing here as raindrops dominate the backscatter. One other thing to note is the higher correlation coefficient and lower ZH and ZDR closer to the radar site in northern VA. The lack of a melting signature here is because the radar beam is closer to the ground at these locations and thus propagates beneath the melting layer; once the radar beam is higher off the ground at locations farther from the radar site, the polarimetric melting layer signature is visible. When the beam gets high enough, it samples above the melting layer and the signature disappears, as correlation coefficient becomes close to 1 and ZH and ZDR values decrease.

I measured 2.5" here with light snow currently. The amount of sleet mixing in last night definitely contributed to the south-north snowfall gradient in Howard County.

Mostly sleet here in southern Howard County with a coating of frozen precip on the ground.

Light snow here.