The Sierra Nevada range is actually quite a ways north of San Diego County--you're now in the Peninsular Ranges that extend all the way down Baja. The rocks and overall shape of the ranges are similar to those of the Sierra, though--granitic rocks of Mesozoic age, and a steep eastern face with a much gentler western slope. Jacumba is still considered high desert--you don't get out of that probably until you get over La Posta Summit at over 4000'.
The desert, including Borrego Springs, did get some very heavy rain and thunderstorms this morning and afternoon--some places up to three inches. That's very unusual for March, but flash flooding is occurring very early this year! The radar shows Jacumba also picked up quite a bit of rain. There also may be another significant, but colder, storm coming on Monday.